APV_Corrosion_Handbook_1035_01_08_2008_US

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Corrosion Handbook



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For more than 75 years, APV has provided customers worldwidewith the latest advancements in heat exchanger technology.Today, we continue to lead the industry with our world renownedstate-o-the-art technology, unsurpassed process knowledge,and an unwavering commitment to our customers.

APV has evolved and grown over the years to better meet the changing needs o ourcustomers and their industries. The irst commercially successul plate-and-rame heatexchanger was introduced in 1923 by the Aluminum Plant and Vessel Company Ltd.,which became known as APV. The irst Paralow Plate Heat Exchanger, constructed o

cast gunmetal plates and enclosed within a crude rame, set the standard or today’scomputer-designed thin metal plate.

Our vision or the uture is rooted in a long standing

tradition o excellence and commitment to progress. Westrive to oer customers the highest quality productsand services today, tomorrow and beyond.

The cover photo depicts galvanically induced corrosion o 316 stainless steelcaused by the deposition o active carbon used as a decolorizing agent or thesugar solution in contact with the metal.

Stainless SteelTie Bar Frame

Carbon Steel

Tie Bar Frame2

APV ounder: Dr. Richard Seligman



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The APV Corrosion Handbook is an essential tool in helping youminimize the impact o corrosion within your heat transer applications.

The handbook provides a detailed look at the various types o corrosion,the eects o the most common corrosive agents with respect to avariety o materials and an overview o how to cost eectively select thematerials o construction most suited or your particular application.

Materials Science and Corrosion Prevention

Section Contents Page1 Materials o Construction 4

2 Selecting Materials o Construction 93 Types o Corrosion 104 Corrosion o Speciic Environments 265 Corrosion by Insulating Materials 356 Corrosion o Rubbers 37

For additional inormation speciic to your needs, pleasecontact APV at (919) 735-4570 or (800) 207-2708.



1. MATERIALS OFCONSTRUCTION

1.1 The “stainless steels”Many people reer to this range oalloys as austenitic stainless steels,18-8s, 18-10s, etc. without a ullappreciation o what is meant by theterminology. It is worth devoting aew paragraphs to explain the basicmetallurgy o stainless steels.In 1913 Harry Brearly discovered

that the addition o 11% chromium tocarbon steel would impart a good levelo corrosion and oxidation resistance.By 1914, these corrosion resistingsteels had become commerciallyavailable. Brearly pioneered the irstcommercial use o these steels or cutlery, and also coined the name “stainless steels.”For metallurgical reasons, which are outside the scope o this paper, these wereknown as “erritic steels” due to their crystallographic structure. Unortunately, they

lacked the ductility to undergo extensive abrication, and they could not be welded.Numerous workers tried to overcome these deiciencies by the addition o other alloyingelements, to produce a material where the errite was transormed to austenite (anothermetallurgical phrase) – that was stable at room temperature. Sot stainless steelsthat were ductile both beore and ater welding were developed in Sheield, England(then the heart o the British steel industry) – exploiting scientiic work undertaken inGermany. This new group o steels was based on 18% chromium steel, to which nickelwas added as a second alloying element. These were termed the “austenitic stainlesssteels.” The general relationship between chromium and nickel necessary to maintain

a ully austenitic structure is shown in Figure 1. The optimum combination is 18%chromium, 8% nickel – hence the terminology 18/8s.Probably the next major advance in the development o stainless steels was thediscovery that relatively small additions o molybdenum had a pronounced eect on theircorrosion resistance, greatly enhancing their ability to withstand the eects o mineralacids and other corrodents such as chloride solutions. Needless to say, rom these earlydevelopments, there has been tremendous growth in the production acilities and thenumber o grades o stainless steel available. Table 1 lists some o the more commonlyavailable grades while Figure 2 illustrates how the basic 18/8 composition is modiied

to enhance speciic physical or chemical properties.

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Materials Science and Corrosion Prevention

30

20

10

0

0 10 20 30

Austenite

Ferritic or intermediate

structure

% N

i

% Cr

Fig. 1: Graph showing the various combinationso chromium and nickel that orm austeniticstainless steels (Re. 1).



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NICKEL, CHROMIUM

IRON ALLOYS

NICKEL, CHROMIUM,IRON (MOLYBDENUM,COPPER, NIOBIUN)

ALLOYS

304L

316L

317L

317

316

304(18/8)

309, 310,314, 330

DUPLEXSTAINLESS

STEELS (329)

PRECIPITATIONHARDENINGSTAINLESS

STEELS

AUSTENITICIRON, NICKEL,MANGANESE,NITROGEN,STAINLESS

STEELS

303, 303, SELENIUM

347

321

Add niobium & titaniumto reduce

sensitization

Add titaniumto reduce

sensitization

Lower carbonto reduce

sensitization

Add nickel, molybdenum,copper, niobium

for corrosionresistance in reducing

environments

Add more molybdenumfor pitting resistance

Add molybdenumfor pitting resistance

Add chromium and nickelfor strength and

oxidation resistance

Add nickel forcorrosion resistance in hightemperature environments

Add sulphur toselenium for machinability

(corrosion resistancereduced)

Increase chromiumlower nickel for

special properties

Add copper titanium,aluminum, lower nickel

for precipitationhardening (corrosionresistance reduced)

Add manganese andnitrogen, lowernickel for higher

strength (corrosionresistance reduced)

ALLOY UNS COMPOSITION %NO. CARBON MANGANESE SILICON CHROMIUM NICKEL MOLYBDENUM OTHERS

302 S30200 0.15 2.00 1.00 16.0 — 18.0 6.0 — 8.0 — Sulur 0.030

Phosphorus 0.045304 S30400 0.08 2.00 1.00 18.0 — 20.0 8.0 — 10.5 — Sulur 0.030

Phosphorus 0.045

304L S30403 0.03 2.00 1.00 18.0 — 20.0 8.0 — 12.0 — Sulur 0.030Phosphorus 0.045

316 S31600 0.08 2.00 1.00 16.0 — 18.0 10.0 — 14.0 2.0 — 3.0 Sulur 0.030Phosphorus 0.045

316L S31603 0.03 2.00 1.00 16.0 — 18.0 10.0 — 14.0 2.0 — 3.0 Sulur 0.030Phosphorus 0.045

317 S31700 0.08 2.00 1.00 18.0 — 20.0 11.0 — 15.0 3.0 — 4.0 Sulur 0.030Phosphorus 0.045

317L S31703 0.03 2.00 1.00 18.0 — 20.0 11.0 — 15.0 3.0 — 4.0 Sulur 0.030Phosphorus 0.045

321 S32100 0.08 2.00 1.00 17.0 — 19.0 9.0 — 12.0 — Sulur 0.030Ti<5xCarbon

347 S34700 0.08 2.00 1.00 17.0 — 19.0 9.0 — 13.0 — Sulur 0.030Cb+Ta<10xCarbon

Table 1: Composition o some o the more commonly used austenitic stainless steels. Unless indicated otherwise, allvalues are maximum.

Fig. 2: Outline o some compositional modiications o 18/8 austenitic stainless steel to produce specialproperties (Re. 2).



In spite o the plethora o stainless steels available, grades 304 and 316 have, andcontinue to be, the workhorses or abrication o dairy and ood processing equipment.

Although 316 stainless steel oers excellent resistance to a wide range o chemicaland non-chemical environments, it does not oer immunity to all. In the case o the oodindustry, these are notably anything containing salt, especially low pH products. Therewas, thereore, a demand by industry to develop more corrosion resistant materials,and these are inding an increased use in the ood industry or certain speciicprocessing operations.

1.2 Super stainless steels and nickel alloysThe super stainless steels are a group o alloys which have enhanced levels o

chromium, nickel, and molybdenum, compared to the conventional 18/8s. The majorconstituent is still iron, hence the classiication under the “steel” title. Still, urtherincreases in the three aorementioned alloying elements result in the nickel alloys.(The classiication o an alloy is generally under the heading o the major constituent.)There are a large number o these alloys, but those o primary interest to the oodindustry are shown in Table 2, together with their compositions. In general terms, it willbe noted that the increase in nickel content is accompanied by an increase inchromium and molybdenum. As previously stated, this element is particularly eective inpromoting corrosion resistance.

Just like insurance, you only get what you pay or – and generally speaking, the higherthe corrosion resistance, the more expensive the material. In act, the dierentialbetween type 304 stainless steel and a high nickel alloy may be as much as 20 times,depending on the market prices or the various alloying elements – which luctuatewidely with the supply and demand position.

1.3 AluminumHigh purity grades o aluminum (±99.5%) and its alloys still are preerred or some oodand pharmaceutical applications due to the reasonable corrosion resistance o themetal. This resistance is attributable to the easy and rapid ormation o a thin,continuous, adherent oxide ilm on exposed suraces. This oxide ilm, in turn, exhibits a

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Table 2: Composition o some o the more commonly used wrought super stainless steels and nickel alloys.Unless indicated otherwise, all values are maximum.

904L N08904 0.02 2.0 0.70 19.0 — 21.0 24.0 — 26.0 4.2 — 4.7 Cu 1.2 — 1.7

Avesta S31254 0.02 2.0 0.80 19.5 — 20.5 17.5 — 18.5 6.0 — 6.5 Cu 0.5 — 1.0254 SMO N 0.18 — 0.22

Incoloy N08825 0.05 1.0 0.50 19.5 — 23.5 38.0 — 46.0 2.5 — 3.5 Al 0.2825 Ti 0.6 — 1.2

Hastelloy N06030 0.03 1.5 0.80 28.0 — 31.5 Bal. 4.0 — 6.0 Co 5.09G-30 Cu 1.0 — 2.4

Cb+Ta 0.3 — 1.5Fe 13.0 — 17.0

Inconel N06625 0.10 0.50 0.50 23.0 — 28.0 Bal. 8.0 — 10.0 Co 1.0, Fe 5.0Al 0.4, Ti 0.4

Hastelloy N10276 0.02 1.0 0.08 14.5 — 16.5 — 15.0 — 17.0 W 3.0 — 4.5,C-276 V 0.35

ALLOYUNS.NO. CARBON MANGANESE SILICON

COMPOSITION %CHROMIUM NICKEL MOLYBDENUM OTHERS



good corrosion resistance to many oodstus; and it is reported that ats, oils, sugar,and some colloids have an inhibitory or sealing eect on these ilms (Re. 3).

As aluminum salts ormed by corrosion are colorless, tasteless, and claimed to benon-toxic – the metal is easy to clean, inexpensive, light, and has a high thermalconductivity. It still is used quite extensively in certain areas o ood manuacture anddistribution. However, in recent years, the claim o non-toxicity is being questioned as ahigh dietary incidence has been implicated in Alzheimer’s disease (senile dementia) withcompounds o aluminum (aluminosilicates) being ound in the brain tissue osuerers (Re. 4). However, the case is ar rom proven, and it is not clear i theincreased levels o aluminosilicates are due to a high intake o aluminum per se or otheractors such as dietary deiciency o calcium.

For many years, aluminum was extensively used or containment vessels in the dairy andbrewing industry. It was Richard Seligman who ounded the then Aluminum Plant andVessel Company (now APV, An SPX Brand) to exploit the technique o welding thismaterial or the abrication o ermenting vessels in the brewing industry (Re. 5).Many o these original vessels are still in use in some o the smaller, privately ownedbreweries in the United Kingdom.Although large ermenting vessels and storage tanks now tend to be abricated romstainless steels, there is still widespread use o aluminum or beer kegs, beer cans, anda miscellany o small-scale equipment where the resistance o aluminum is such that it

imparts no charge or modiication o lavor, even ater prolonged storage.While still used or holding vessels and some equipment when processing cider, wines,and perry, prolonged contact is inadvisable due to the acidity o the sulphites employedas preservatives or these products – inadvisable, that is, unless the surace o themetal has been modiied by anodizing or has been protected with a lacquer.In the manuacture o preservatives, aluminum is still employed or boiling pans,with the presence o sugar appearing to inhibit any corrosion. In the ield o apiculture, ithas even been used or making preabricated honeycombs which the beesreadily accepted.

Extensive use is made o aluminum and the alloys in the baking industry or baking tins,kneading troughs, handling equipment, etc.In other areas o ood manuacture and preparation, the use o aluminum virtuallyextends over the whole ield o activity – butter, margarine, table oils and edible ats,meat and meat products, ish and shellish, certain sorts o vinegar, mustards, spices;the list is almost endless.No mention has so ar been made o the application o this metal in the dairyindustry. It still has limited application mostly in the ield o packaging, e.g., bottle caps,wrapping or cheese, butter, carton caps or yogurt, cream, etc.

The uses o aluminum in the ood industry so ar mentioned have tended to be orequipment used in batch operation, hand utensils, and packaging. There are probablythree major actors which have mitigated against its more widespread use, not only inthe dairy industry but in brewing and many other branches o ood processing.1. Modern, highly automated plants operating on a continuous or semi-continuous basis

employ a wide variety o materials o construction. Because o the position in the

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electrochemical series (to be discussed later), aluminum and its alloys are susceptibleto galvanic corrosion when coupled with other metals.

2. The commercial availability o stainless steels, their ease o abrication, strength, easeo maintenance, appearance, and proven track record o reliability.

3. Since modern plants operate on a semi-continuous basis with much higher levels oouling, cleaning regimes require strongly alkaline detergents to which aluminum hasvirtually zero corrosion resistance.

1.4 Copper and tinned copperCopper and tinned copper were extensively used in ormer times due to their excellentthermal conductivity (eight times that o stainless steel), their ductility, ease o

abrication, and reasonable level o corrosion resistance. However, the demise o copperas a material o construction is largely attributable to the toxic nature o the metal andits catalytic activity in the development o oxidative rancidity in ats and oils. Even atthe sub-part per million level, copper in vegetable oils and animal ats rapidly causesthe development o o-lavors. In equipment where high levels o liquid turbulence areencountered, e.g., plate heat exchanger or high velocity pipe lines, copper is subjectto erosion. Nevertheless, there is an area o the beverage industry where copper is stillthe only accepted material o construction, i.e., pot stills or scotch and Irish whiskeyproduction. It is also used in the distillation o spirits such as rum and brandy. A lot o

old copper brewing equipment, such as ermenting vessels and wort boilers, is still inuse throughout the world. An interesting observation is that even though wort boilers inmodern breweries are abricated rom stainless steel, they are still known as “coppers,”and U.K. cratsmen abricating stainless steel are still known as “coppersmiths.”

1.5 Titanium

There are certain areas o the ood industry, especially in equipment involving heattranser, where stainless steels are just not capable o withstanding the corrosive eectso salty, low pH environments. More and more, ood processors are accepting the use

o titanium as an alternative, in the ull knowledge that it oers corrosion immunity to themore aggressive oodstus and provides a long term solution to what was an on-goingproblem with stainless steels. Although relatively expensive (six to seven times the costo stainless steel), being a low density material osets this price dierential or the rawmaterial by almost hal. It is ductile and abricable using normal techniques, althoughwelding it does require a high degree o expertise.

1.6 Other metalsTin, in the orm o tin plate, is extensively used in the canning industry where its long

term corrosion resistance to a wide range o ood acids makes it a material “parexcellence” or this purpose.Cadmium, used as a protective coating or carbon steel nuts and bolts, was avored atone time. However, the high toxicity o cadmium compounds has come under increasedscrutiny rom many health regulatory bodies, and now cadmium plated bolting is notpermitted in ood actories. Denmark and Sweden have totally banned the import ocadmium plated components into their countries and many other countries are likely toollow suit.

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Lead and lead-containing productsare generally not acceptable or ood

contact suraces, although somecodes o practice permit the use olead-containing solder or capillarypipeline joints on water supplies andservice lines.

2. SELECTING MATERIALS OFCONSTRUCTION (Ref. 6)Designing equipment is a multi-

discipline exercise involvingmechanical engineers, materials/corrosion engineers, stressing experts, dratsmen, etc.The corrosion engineer has an important role in this team eort insoar as it is his/her

job to ensure the materials speciied will oer a corrosion resistance which is adequateor all the environmental conditions likely to be encountered during normal operation othe equipment. A piece o equipment which prematurely ails by corrosion is as badlydesigned as one in which the materials have been over-speciied. Unortunately, all toooten the unctional requirements or a piece o equipment are analyzed in a somewhatarbitrary manner. And the basic cost o the material oten tends to outweigh other

equally important considerations. Figure 3 shows the primary criteria which must beconsidered in the initial selection process.• Corrosion resistanceFor any processing operation, there will be a range o materials which will oer acorrosion resistance which is adequate (or more than adequate) or a particular job.When considering corrosion resistance, the operational environment is the obvious one.But, the other point must be whether the material will also oer corrosion resistance tothe chemicals used or cleaning and sanitizing.• Cost

Many o the materials originally considered will be eliminated on the grounds o theirhigh cost. For example, there is no point in considering a high nickel alloy when astandard 300 series stainless steel, at a lower cost, will be perectly satisactory.• AvailabilityAvailability is a less obvious eature o the material selection process. Many steelproducers will require a minimum order o, say, three tons or a non-standardmaterial. Clearly, the equipment manuacturer is not going to buy this large quantitywhen the job that is to be done may only require the use o one ton o material.• Strength

Strength is a actor which is taken into account at the design stage, but as with allthe others, cannot be considered in isolation. For example, many o the new strongerstainless steels, although more expensive on a ton-or-ton basis than conventionalstainless steels, are less expensive when considered on a strength/cost ratio.• FabricabilityThere is little point in considering materials which are either unweldable (or unabricable)or can only be welded under conditions more akin to a surgical operating theater than ageneral engineering abrication shop.

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CORROSIONRESISTANCE

COST

AVAILABILITY

STRENGTHMATERIAL

FABRICABILITYAPPEARANCE

MAINTENANCE

Fig. 3: Materials selection criteria.



• AppearanceAppearance may or may not be an important requirement. Equipment located outside

must be resistant to environmental weathering, and thereore, may require theapplication o protective sheathing – which could double the basic material cost.• MaintenanceIs the equipment to be essentially maintenance-ree or is some maintenance, such asperiodic repainting, tolerable? How long will the equipment operate without the need ormajor servicing?When all these interrelated criteria have been considered, the long list o possiblestarters will have been reduced to maybe one or two. Also, through the selectionprocess some o those materials initially rejected due to their high cost, or example,

may have to be reconsidered because o other actors.

3. TYPES OF CORROSION3.1 Defining corrosionBeore embarking on a discussiono the various orms o corrosion, itis worthwhile to consider the exactmeaning o corrosion. There are severaldeinitions o corrosion. For example,

Fontana (Re. 7) deines it as extractivemetallurgy in reverse using thediagram, shown in Figure 4, to illustratethe point.A more general and descriptivedeinition is “it is the deteriorationor destruction o a material throughinteraction with its environment.” This covers all materials o construction includingrubber, plastics and metal. However, the primary object o this paper is to deal with

corrosion o metals, in particular stainless steels, and how this corrosion can beclassiied.There are two basic orms o corrosion – wet and dry. Dry corrosion is concerned withthe oxidation o metals at high temperatures and is outside the scope o this text.Wet corrosion occurs in aqueous solutions, or in the presence o electrolytes, and isan electrochemical process. It should be noted that the “aqueous” component o thesystem may be present in only trace quantities, e.g., present as moisture; the classicalexample being the corrosion o steel by chlorine gas. In act, steel is not corroded bychlorine since steel is the material used or storing liquid chlorine. However, in the

presence o even trace quantities o moisture, chlorine rapidly attacks steel – and orthat matter, most metals.The corrosion o metals involves a whole range o actors. These may be chemical,electrochemical, biological, metallurgical, or mechanical – acting singly or conjointly.Nevertheless, the main parameter governing corrosion o metals is related toelectrochemistry. Electrochemical principles, thereore, are the basis or a theoreticalunderstanding o the subject. In act, electrochemical techniques are now the standard

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OREMINE

BLASTFURNACE

ROLLINGMILL

CARMANUFACTURER

RAIN, SNOW& SALT

RUST

Extractive metallurgy in reverse

Fig. 4



method or investigating corrosion, although the “weight loss” approach still providesinvaluable data. It is not proposed to discuss, in depth, the electrochemical nature o

corrosion – but should urther inormation be required, several excellent texts areavailable (Re. 7, 8).

3.2 Forms of corrosionWet Corrosion can be classiied under any o eight headings:• Galvanic or bimetallic corrosion• Uniform or general attack• Crevice corrosion• Pitting corrosion• Intergranular corrosion• Stress corrosion cracking• Corrosion fatigue• Selective corrosion (castings and free machining stainless steels)

3.2.1 Galvanic corrosion

When two dissimilar metals (or alloys) are immersed in a corrosive or conductivesolution, an electrical potential or potential dierence usually exists between them. I

the two metals are electrically connected, then, because o this potential dierence, alow o current occurs. As the corrosion process is an electrochemical phenomenon anddissolution o a metal involves electron low, the corrosion rates or the two metals areaected. Generally, the corrosion rate or the least corrosion resistant is enhanced whilethat o the more corrosion resistant is diminished. In simple electrochemical terms, theleast resistant metal has become anodic and the more resistant cathodic. This, then, isgalvanic or dissimilar metal corrosion.The magnitude o the changes in corrosion rates depends on the so-called electrodepotentials o the two metals; the greater the dierence, the greater the enhancement or

diminution o the corrosion rates. It is possible to draw up a table o some commercialalloys which ranks them in order o their electrochemical potential. Such a table isknown as the galvanic series. A typical one, as shown in Table 3, is at the Harbor Island,NC, test acility. This galvanic series relates to tests in unpolluted sea water, althoughdierent environments could produce dierent results and rankings. When coupled,individual metals and alloys rom the same group are unlikely to show galvanic eectswhich will cause any change in their corrosion rates.The problem o dissimilar metal corrosion (being relatively well understood andappreciated by engineers) is usually avoided in plant construction, and in the author’s

experience, ew cases have been encountered. Probably the most common orm ounintentional galvanic corrosion is on service lines where brass ittings are used on steelpipelines – the steel suering an increase in corrosion rate at the bimetallic junction.

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One o the worst bimetallic combinations is aluminum and copper. An example o thisis in relation to aluminum milk churns used to transport whey rom Gruyere (a cheesemanuacturer in Switzerland), where copper is used or the cheese-making vats andthe whey picks up traces o this metal. The eect on the aluminum churns, which areinternally protected with lacquer that gets worn away through mechanical damage, ispretty catastrophic.

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Table 3: The galvanic series o some commercial metals and alloys in clean sea water.

PlatinumGoldGraphiteTitaniumSilver

Chlorimet 3 (62 Ni, 18 Cr, 18 Mo)Hastelloy C (62 Ni, 17 Cr, 15 Mo)

18-8 Mo stainless steel (passive)18-8 stainless steel (passive)Chromium stainless steel 11-30% Cr (passive)

Inconel (passive) (80 Ni, 13 Cr, 7 Fe)Nickel (passive)

Silver solder

Monel (70 Ni, 30 Cu)Cupronickels (60-90 Cu, 40-10 Ni)Bronzes (Cu-Sn)CopperBrasses (Cu-Zn)

Chlorimet 2 (66 Ni, 32 Mo, 1 Fe)Hastelloy B (60 Ni, 30 Mo, 6 Fe, 1 Mn)

Inconel (active)Nickel (active)

TinLeadLead-tin solders

18-8 Mo stainless steel (active)18-8 stainless steel (active)

Ni-Resist (high Ni cast iron)Chromium stainless steel, 13% Cr (active)

Cast iron

Steel or iron

2024 aluminum (4.5 Cu, 1.5 Mg, 0.6 Mn)CadmiumCommercially pure aluminum (1100)ZincMagnesium and magnesium alloys

Noble or Cathodic

Active or Anodic



Another somewhat unique example o galvanic corrosion is related to a weld repair on a304 stainless steel storage vessel. Welding consumables containing molybdenum had

been employed to eect the repair. Although it is unusual or the steels to be suicientto initiate galvanic corrosion, the environmental actors in this particular case wereobviously such that corrosion was initiated (Figure 5). As stated, this is somewhatunique and it is not uncommon or 316 welding consumables to be used or welding304 stainless steel with no adverse eects. As a practice, however, it is to bedeprecated and the correct welding consumables should always be employed. Not allgalvanic corrosion is bad; galvanic corrosion is used extensively to protect metal andstructures by the use o a sacriicial metal coating. A classic example is the galvanizingo sheet steel and ittings, the zinc coating being applied not so much because it

doesn’t corrode, but because it does. When the galvanizing ilm is damaged, the zincgalvanically protects the exposed steel and inhibits rusting. Similarly, sacriicial anodesare itted to domestic hot water storage tanks to protect the tank.

3.2.2 Uniform or general attack As the name implies, this orm o corrosion occurs more or less uniormly over thewhole surace o the metal exposed to the corrosive environment. It is the most commonorm o corrosion encountered with the majority o metals, a classic example being therusting o carbon steel. Insoar as the corrosion occurs uniormly, corrosion rates are

predictable and the necessary corrosion allowances are built into any equipment. In thecase o stainless steels, this orm o corrosion is rarely encountered. Corrodents likely toproduce general attack o stainless steel are certain mineral acids, some organic acidsand high strength caustic soda at concentrations and temperatures well in excess othose ever likely to be ound in the ood industry. The same remark applies to cleaningacids such as nitric, phosphoric and citric acids – but not or suluric or hydrochloricacids – both o which can cause rapid, general corrosion o stainless steels. Hence,they are not recommended or use, especially where corrosion would result in adeterioration o the surace inish o process equipment.

The behavior o both 304 and 316 stainless steels when subjected to some o themore common acids that are encountered in the ood industry is graphically illustratedby Figure 6. These iso-corrosion graphs, i.e., lines which deine the conditions otemperature and acid concentration which will produce a constant corrosion rateexpressed in mils (0.001”) or mm loss o metal thickness per year, are extensively usedby corrosion engineers in the material selection process when the orm o corrosionis general attack. They are of no value when the corrosion mode is one of theother forms which will be defined, such as pitting or crevice corrosion.

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Fig. 5: Galvanic corrosion o 304 stainless steelinitiated by a 316 weld deposit. Note thelarge pit associated with the weld splatter.



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HYDROCHLORIC ACID

B.P. CURVE B.P. CURVE

B.P. CURVE B.P. CURVE

B.P. CURVEB.P. CURVE

B.P. CURVEB.P. CURVE

% ACID BY WEIGHT % ACID BY WEIGHT




T E M P E R A T

U R E ( º C )

T E M P E R A T U R E ( º C )

T E M P E R A T U R E ( º

C )






TYPE 304 TYPE 316

NITRIC ACID

TYPE 304 TYPE 316

PHOSPHORIC ACID

TYPE 304 TYPE 316

SULPHURIC ACIDTYPE 304 TYPE 316

120

100

80

60

40

20

120

100

80

60

40

20

0 5 10 15 20 0 5 10 15 20

120

100

80

60

40

20

120

100

80

60

40

20

120

0 20 40 60 80 100 0 20 40 60 80 100

120

00

100100

2020

8080

4040

6060

60 60

40 40

80 80

2020

100 100

120 120

0 0

100 100

20 20

80 80

40 40

6060

60 60

4040

80 80

2020

100 100

<1 MILS/YEAR

<0.03 MM/YEAR

1-5 MILS/YEAR

0.03 - 0.13 MM/YEAR

5 - 30 MILS/YEAR

0.13 - 0.75 MM/YEAR

>30 MILS/YEAR

>0.75 MM/YEAR

Fig. 6: Corrosion resistance o 304 and 316 stainless steels to mineral acids (reproduced by permissiono British Steel Pic).



3.2.3 Crevice corrosion

This orm o corrosion is an intense local attack

within crevices or shielded areas on metalsuraces exposed to corrosive solutions. It ischaracteristically encountered with metals andalloys which rely on a surace oxide ilm orcorrosion protection, e.g., stainless steels,titanium, aluminum, etc.The crevices can be inherent in the design o theequipment (e.g., plate heat exchangers) orinadvertently created by a bad design. Crevice

corrosion can be initiated at metal to non-metallic sealing aces. Any non-metallic material which is porous and used as a gasket,or example, is particularly good (or bad!) or initiating this orm o attack. Fibrousmaterials which have a strong wicking action are notorious in their ability to initiatecrevice attack. Similarly, materials which have poor stress relaxation characteristics,i.e., have little or no ability to recover their original shape ater being deormed, are alsocrevice creators – as are materials which tend to creep under the inluence o appliedloads and/or at elevated temperatures. Although used or gasketing, P.T.F.E. suersboth these deiciencies. On the other hand, elastomeric materials are particularly good

insoar as they exhibit elastic recovery and have the ability to orm a crevice-ree seal.However, at elevated temperatures, may rubbers harden. In this condition, they suerthe deiciencies o non-elastomeric gasketing materials.Artiicial crevices can also be created by the deposition o scale rom one o theprocess streams to which the metal is exposed. It is necessary, thereore, to maintainood processing equipment in a scale-ree condition – especially on suraces exposedto service luids such as hot/cold water, cooling brines, etc. – which tend to beoverlooked during plant cleaning operations.Much research has been done on the geometry o crevices and the inluence o this on

the propensity or the initiation o crevice corrosion (Re. 9). However, in practical terms,crevice corrosion usually occurs in openings a ew tenths o a millimeter or less, andrarely is encountered where the crevice is greater than 2 mm (0.08”).Until the 1950s, crevice corrosion was thought to be due to dierences in metal ion oroxygen concentration within the crevice and its surroundings. While these are actorsin the initiation and propagation o crevice corrosion, they are not the primary cause.Current theory supports the view that through a series o electrochemical reactions andthe geometrically restricted access into the crevice migration o cations – chloride ionsin particular – occurs. This alters the environment within, with a large reduction on pH

and an increase in the cations by a actor as much as ten. The pH value can all rom avalue o, say, seven in the surrounding solution to as low as two within the crevice. Ascorrosion is initiated, it proceeds in an autocatalytic manner with all the damage andmetal dissolution occurring within the crevice. Corrosion results in signiicant loss ometal under the surace o site o initiation. As a result, deep and severe cutting othe metal occurs (see Figure 8). The time scale or initiation o crevice corrosion canvary rom a ew hours to several months, and once initiated, can very rapidly progress.Stopping the corrosion process can be extremely diicult as it is necessary to remove

15

Fig. 7: Crevice corrosion at the interplatecontact points o a heat exchanger plate.



all the trapped reactants andcompletely modiy the occluded

environment. The diiculty oattaining this will be appreciatedby reerence to Figure 8, wherethe entrance to the crevice is only0.5 mm (0.020”).While methods or combating theonset o crevice corrosion canbe deduced rom the oregoingtext, a reiteration o some o the

more important precautions is asollows:• Good quality, crevice free,

welded joints are alwayspreerable to bolted joints.

• Good equipment design (well designed, gasket-sealing faces) which avoidsunintentional crevices and does not permit the development o stagnant regions.

• Frequent inspection of equipment and removal of surface deposits.• Use of good quality, rubber gaskets rather than absorbent packings.

• Good gasket maintenance; replacement when hardened or damaged.However, certain pieces o equipment are by virtue o their design highly creviced.In such cases, it is necessary to recognize the potential corrosion risk and select thematerials o construction which will resist the initiation o crevice corrosion by theenvironment. Similarly, cleaning and sanitizing regimes must be developed to avoid theonset o attack.In the case o stainless steels, although there are several ionic species which will initiatethe attack, by ar the most common are solutions containing chloride. The presence osalt in virtually all oodstus highlights the problem. Low pH values also enhance the

propensity or initiation o attack.Other environmental actors, such as temperature and the oxygen or dissolved aircontent o the process stream, all play a role in the corrosion process.Because the presence o oxygen is a prerequisite or the onset o crevice corrosion(and many other orms o attack), in theoretical terms complete removal o oxygen roma process stream will inhibit corrosion. In practice, however, this is diicult to achieve.Only in equipment where complete and eective deaeration occurs, such as a multipleeect evaporator operating under reduced pressure, will the beneicial eect o oxygenremoval be achieved.

Stainless steels containing molybdenum (316, 317) have a much higher resistanceto crevice corrosion than alloys without this element (304, 321, 347). The higher themolybdenum content, the greater the corrosion resistance. For particularly aggressiveprocess streams, titanium is oten the only economically viable material to oeradequate corrosion resistance.

16

Fig. 8: Photomicrograph o a section through a site o crevice

corrosion. Note the deep undercutting which is typicalo chloride-induced attack on stainless steel.



3.2.4 Pitting corrosionAs the name implies, pitting is a

orm o corrosion which leads tothe development o pits on a metalsurace. It is a orm o extremelylocalized but intense attack,insidious insoar as the actual losso metal is negligible in relationto the total mass o metal whichmay be aected. Nevertheless,equipment ailure by peroration

is the usual outcome o pittingcorrosion. The pits can be smalland sporadically distributedover the metal surace (Figure9) or extremely close together,close enough in act to give theappearance o the metal havingsuered rom a general attack.In the case o stainless steels,

environments which will initiatecrevice corrosion will alsoinduce pitting. As ar as the oodindustry is concerned, it is almostexclusively caused by chloridecontaining media, particularly atlow pH values.Many theories have been developed to explain the cause o the initiation o pittingcorrosion (Re. 10), and the one eature they have in common is that there is

a breakdown in the passive oxide ilm. This results in ionic migration and thedevelopment o an electrochemical cell. There is, however, no uniied theory whichexplains the reason or the ilm breakdown. Evans (Re. 11), or example, suggests thatmetal dissolution at the onset o pitting may be due to a surace scratch, an emergingdislocation or other deects, or random variations in solution composition. However,propagation o the pit proceeds by a mechanism similar to that occurring with crevicecorrosion. Like crevice corrosion, the pits are oten undercut and on vertical suracesmay assume an elongated morphology due to gravitational eects (Figure 10).The onset o pitting corrosion can occur in a matter o days but requently requires

several months or the development o recognizable pits. This makes the assessment othe pitting propensity o a particular environment very diicult to determine, and thereare no short-cut laboratory testing techniques available. Methods and tests solutionsare available to rank alloys, the best known and most requently quoted being ASTMStandard G48 (Re. 12), which employs 6% erric chloride solution. Another chemicalmethod involving erric chloride determines the temperature at which the solution willcause pitting within a 24-hour period, the results being expressed as the critical pittingtemperature or CPT (Re. 13). However, as stated, both o these methods are used

17

Fig. 9: Pitting corrosion o a stainless steel injector caused bythe presence o a hydrochloric acid in the steam supply.

Fig. 10: Elongated pitting attack on a 316 stainless steel heatexchanger plate.



to rank the susceptibility o a range o alloys rather than deine the perormance o a

material in a service environment. Electrochemical methods have also been used.As with crevice corrosion, alloy composition has a proound eect on the resistance oa material to a pitting attack. Greene and Fontana (Re. 14) summarized the eect ovarious elements as shown in Table 4.

3.2.5 Intergranular corrosionA act not oten appreciated is that metals and alloys have a crystalline structure.However, unlike crystalline solids such as sugar or salt, metallic crystals can bedeormed or bent without racturing; in other words, they are ductile. In the molten

state, the atoms in a metal are randomly distributed but on cooling and solidiication,they become arranged in crystalline orm. Because crystallization occurs at manypoints in the solidiication process, these crystals or grains are randomly orientated andthe region where they meet are grain boundaries. In thermodynamic terms, the grainboundaries are more susceptible to corrosion attack because o their higher ree energy.Although, in practice the ree energy o the grain boundary and the main crystals orgrains in a homogeneous alloy are too small to make a signiicant dierence. However,when the metal or alloy has a heterogeneous structure, preerential attack at or adjacentto the grain boundaries can occur.

This is intergranular corrosion asshown in Figure 11.When austenitic stainless steelsare heated to and held in thetemperature range o 600-900°C(1110-1650°F), the materialbecomes sensitized andsusceptible to grain boundarycorrosion. It is generally agreed

that this is due to chromiumcombining with carbon toorm chromium carbide, whichis precipitated at the grainboundaries. The net eect is thatthe metal immediately adjacent to thegrain boundaries is denuded o chromium and instead o having a composition o, say18% chromium and 8% nickel, it may assume an alloy

18

Fig. 11: Scanning electron micrograph o the surace osensitized stainless steel showing preerential attackalong the grain boundaries.

Table 4: The eect o alloying on pitting resistance o stainless steel alloys.

Effect on Effect onElement Pitting Resistance Element Pitting Resistance

Chromium Increases Molybdenum Increases

Nickel Increases Nitrogen Increases

Titanium/ No eect in media other Sulphur DecreasesNiobium than erric chloride (and selenium)

Silicon Decrease or increase Carbon Decreases i presentdepending on the as grain boundaryabsence or presence o precipitatesmolybdenum



composition where the chromium content is reduced to 9% or even lower. As such, thiszone depleted in chromium bears little similarity to the main metal matrix and has lost

one o the major alloying elements on which it relied or its original corrosion resistance.The lowering o corrosion resistance in this zone is so great that sensitized materials aresubject to attack by even mildly corrosive environments.As supplied rom the steel mills, stainless steels are in the so-called solution annealedcondition, i.e., the carbon is in solution and does not exist as grain boundary chromiumcarbide precipitates. During abrication where welding is involved, the metal adjacentto the weld is subjected to temperatures in the critical range (600-900°C/1110-1650°F) where sensitization can occur. Thereore, this zone may be susceptible to thedevelopment o intergranular carbide precipitates. Because the ormation o chromium

carbides is a unction o time, the longer the dwell time in the critical temperature zone,the greater the propensity or carbide ormation. Hence, the problem is greatest withthicker metal sections due to the thermal mass and slow cooling rate.By heating a sensitized stainless steel to a temperature o 1050°C (1950°F), thecarbide precipitates are taken into solution. By rapidly cooling or quenching the steelrom this temperature, the original homogeneous structure is re-established and theoriginal corrosion resistance is restored.The irst stainless steels were produced with carbon contents o up to 0.2%, and assuch, were extremely susceptible to sensitization and in-service ailure ater welding. In

consequence, the carbon levels were reduced to 0.08% which represented thelower limit attainable with steel-making technology then available. Although this movealleviated the problem, it was not wholly successul, particularly when welding thickersections o the metal. Solution annealing o the abricated items was rarely a practicalproposition and there was a need or a long term solution. It was shown that titanium orniobium (comumbium) had a much greater ainity or the carbon than chromium, and byadditions o either o these elements, the problem was largely overcome.The titanium or niobium carbides which are ormed remain dispersed throughout themetal structure rather than accumulating at the grain boundaries.

Grade 321 is a type 304 (18Cr, 8Ni) with titanium added as a stabilizing element, whilegrade 347 contains niobium. By ar, the most commonly used is 321, grade 347 beingspeciied or certain chemical applications.Modern steel-making techniques such as AOD (air-oxygen decarburization) weredeveloped to reach even lower levels o carbon, typically less than 0.03%, to producethe “L” grades o stainless steel. These are commercially available and routinelyspeciied where no sensitization can be permitted. With these advances in steel-makingtechnology, even the standard grades o stainless steels have typical carbon levelso 0.04/0.05%, and generally speaking, are weldable without risk o shromium

carbide precipitates at metal thicknesses up to 6 mm (1/4”). Above this igure orwhere multipass welding is to be employed, the use o a stabilized or “L” grade isalways advisable.

19



3.2.6 Stress corrosion crackingOne o the most insidious orms o

corrosion encountered with the austeniticstainless steels is stress corrosioncracking (SCC). The morphology othis type o ailure is invariably a ineilementous crack which propagatesthrough the metal in transgranular mode.Frequently, the crack is highly branched asshown in Figure 12, although sometimesit can assume a single crack orm. Factors

such as metal structure, environment,and stress level have an eect on crackmorphology. The disturbing eature oSCC is that there is virtually no loss ometal, and requently it is not visible bycasual inspection and is only apparentater peroration occurs. Some claim thatas much as 50% o the ailures o stainlesssteel are attributable to this cause.

Another characteristic o SCC in stainlesssteels is that once detected, repair by welding is extremely diicult. Crack propagationrequently occurs below the surace o the metal and any attempt to weld repair resultsin the crack opening up and running ahead o the welding torch. The only practicalmethod o achieving a satisactory repair is to completely remove the aected area witha 15-25 cm (six to nine inches) allowance all around the area o visible damage andreplace the section. Even then, there is no guarantee that the damaged zone has beenentirely removed.In most cases, there are three prerequisites or the initiation o SCC.

• Tensile stressThis may be either residual stress rom abricating operations or applied through thenormal operating conditions o the equipment. Furthermore, it has been observed that acorrosion pit can act as a stress raiser and a nucleation site or SCC.• Corrosive speciesAlthough there are a number o ionic compounds which will act as the corrodent, in theood industry this invariably is the chloride ion. High strength caustic soda at elevatedtemperatures will also induce SCC, but the concentrations and temperatures requiredare well in excess o those ever likely to be encountered. Furthermore, the crack

morphology is intergranular rather than transgranular. PH also plays a role, and generallyspeaking, the lower the pH the greater the propensity or SCC.• TemperatureIt generally is regarded by many that a temperature in excess o 60°C (140°F) isrequired or this type o ailure, although the author has seen examples occurring at50°C (122°F) in liquid glucose storage vessels.

20

Fig. 12: Photomicrograph o a typical stresscorrosion crack showing its highly branchedmorphology and transgranular probagation.



In the absence o any one o these prerequisites, the initiation o SCC is eliminated.Thereore, it is worth considering the practical approach to its elimination rom

equipment.Figure 13 is a diagram representing the eect o stress on “time-to-ailure.” As will beseen, by reducing the stress level below a certain critical point, the “time-to-ailure” canbe increased by several orders o magnitude. On small pieces o equipment, residualstress rom manuacturing operations can be removed by stress relie annealing. Forlarge pieces o equipment such asstorage vessels, this approach isclearly impractical. Applied stressis very much a unction o the

operational conditions o theequipment and only by reducingthe stress level, by increasing thethickness o the metal, can this bereduced. However, this is a somewhatimpractical and uneconomic approach.Some (Re. 15) claim that by placingthe surace o the metal undercompressive, rather than tensile stress,

and by shot peening with glass beads,the problem o SCC can be minimizedor eliminated. This, too, is not apractical proposition or many items oood processing equipment.As or the matter o corrosive species,it is questionable i anything can bedone about elimination. With oodstus,or example, this invariably will be the

chloride ion, a naturally occurring oressential additive.Similarly, little can be done in respectto the temperature as this is essentialto the processing operation.As shown by Copson (Re. 16), thetendency or iron-chromium-nickelalloys to ail by SCC in a speciic testmedium (boiling 42% magnesium

chloride solution) is related to thenickel content o the alloy. Figure 14shows this eect and it is unortunatethat stainless steels with a normal 10%nickel have the highest susceptibilityto ailure. Increasing the nickel content

21

80

70

60

50

40

30

20

10

00.1 0.5 1 5 10 50 100 500 1000

FRACTURE TIME, HR.

A P P L I E S S

T R E S S 1

0 0 0

P S I

T Y P E

3 0 4

3 0 4 - L

T Y P E

3 0 5 3 0 9

3 1 6

3 4 7

3 4 7 - L

TYPE

310

314

1000

100

10

0 20 40 60

NICKEL - %

B R E A K I N G T

I M E - H O U R S

M I N I M U M

C R A C K I N

G T

I M E

Fig. 13: Composit curves illustrating the relativeresistance to stress corrosion cracking osome commercial stainless steel in a speciictest solution.

Fig. 14: Stress corrosion cracking o iron, chromium andnickel alloys – the Copson curve. Data pointshave been omitted or clarity.



o the alloy results in a signiicant increase in the time-to-ailure. But o course, this

approach incurs not only the increased cost o the nickel but also the added penaltyo having to increase the chromium to maintain a balanced metallurgical structure. Themore eective approach is by reducing the nickel content o the alloy.A group o stainless steels have been developed which exploit this eature, and althoughtheir composition varies rom producer to producer, they have a nominal composition o20/22% chromium and 5% nickel. Molybdenum may or may not be present, dependingon the environment or which the alloy has been designed. These alloys dier rom theaustenitic stainless steels insoar as they contain approximately 50% errite, hence theirdesignation, austenitic-erritic, or more commonly, duplex stainless steels. It is only with

the advent o modern steel-making technology, particularly in relation to the lowercarbon levels that can be achieved, that these alloys have become a commercially viableproposition. Because o their low carbon content, typically 0.01-0.02%, the originalproblems associated with welding erritic stainless steels and chromium carbideprecipitation have been overcome. The alloys are almost twice as strong as theaustenitic stainless steels, and are ductile and weldable. From a general corrosionstandpoint, they are comparable with, or marginally superior to, their 300 seriesequivalent; but rom an SCC standpoint in test work and rom ield experience, theyoer a resistance order o a magnitude better.

Also now available are ully erritic stainless steels such as grade 444 which contains18% chromium and 2% molybdenum. This alloy contains carbon at the 0.001% level,and thereore, does not suer rom problems o welding which were encountered withthe original erritic steels. Furthermore, stabilizing elements such as titanium and niobiumare also alloying additions which minimize the tendency or intergranular chromiumcarbide ormation. The big disadvantage o these materials is their susceptibility to graingrowth during welding (Figure 15) which makes them extremely sensitive to racture,even at room temperature. Welding sections thicker than 3 mm (1/8”) are not regardedas a practical proposition, and thereore, their use tends to be limited to tubing.

3.2.7 Corrosion fatigueFatigue is not a orm o corrosion because there is no loss o metal, but can beassociated with other orms o localized attack. Because pure atigue is an in vacuophenomenon, a more correct term is corrosion atigue or environmental cracking, whichis the modern expression and takes into account cracking where the corrosive actorhas played a major role on the crack morphology.

22

Fig. 15: Photomicrograph o a weld deposit on a erritic stainless steel. Compare the size o grains in the weldwith those in the parent material on the let.



The primary cause o corrosion atigue is the application o luctuating pressure loads tocomponents which, while o adequate design to withstand normal operating pressures,eventually ail under the inluence o cyclic loading. The components can be o extremelyrigid construction, such as homogenizer block or o relatively light construction, suchas pipework. There are many potential sources o the luctuating pressure, the mostcommon o which are positive displacement pumps (e.g., homogenizer or meteringpumps), rapid acting on-o valves which will produce transient pressure peaks, requentstop-start operations, dead-ending o equipment linked to a illing machine, etc.Generally speaking, atigue cracks are straight, without branching and without ductile

metal distortion o the material adjacent to the crack. The one unique characteristico atigue cracks is that the crack ace requently has a series o conchoidal markingswhich represent the step-wise advance o the crack ront – see Figure 16. Although, asstated, corrosion atigue cracks are generally straight and unbranched – where atiguingconditions are in a potentially corrosive environment – the inluence o the corrosivecomponent may be superimposed on the cracks. This can lead to branching o thecracks, and in the extreme case, the crack may assume a highly branched morphologywhich is almost indistinguishable rom stress corrosion cracking. It is only when thecrack ace is examined under high-power magniication that it is possible to categorize

the ailure mode. An example o this is shown in Figure 17 which has all the eatureso stress corrosion cracking, but the scanning electron micrograph (Figure 18) clearlyshows the step-wise progression o the crack ront.The site or initiation o corrosion atigue is requently a discontinuity in the metalsection o the component. This may be, or example, rom a sharp change in diametero a shat where inadequate radiusing o the diametric change results in a high cyclicstress level through shat rotation, and thus, an initiation point or atigue.

23

Fig. 16: (right) Fatigue cracking o a 1” diameter stainless steel boltshowing the characteristic conchoidal striations on thecrack ace.

Fig. 17: (lower let) Photomicrograph o a atigue crack showing allthe eatures o stress corrosion cracking. Compare this with

Figure 12.

Fig. 18: (lower right) Scanning electron micrograph o the racture ace othe crack shown in Figure 17. Note the step-wise progression othe crack.



A corrosion pit, which under cyclic loading will have a high stress level association, canalso act as a nucleation site (epicenter) or atigue ailure. Because o the corrosive

element, it is sometimes diicult to establish whether the pit and associated corrosionwere the initiating mechanism or the cracking or the result o corrosion superimposedon a crack, the racture orce o which will be in an “active” state and thereore moresusceptible to corrosion processes.The whole subject o atigue and corrosion atigue is complex. However, as ar asood processing equipment is concerned, avoidance o atigue ailure is best achievedby avoiding pulsing and pressure peaks. This requires the use o well engineeredvalving systems and avoiding the use o positive displacement pumps. Where this isimpractical, provisions should be made to incorporate pulsation dampers which will

smooth out the pressure peaks and minimize the risk o atigue ailures.

3.2.8 selective corrosion

3.2.8.1 corrosion of castingsThere are a number o stainless steel components ound in ood process plants, suchas pipeline ittings and pump impellers, which are produced as castings rather thanabricated rom wrought material – notably, the cast equivalents o grade 304 (CF8)AND 316 (CF8M). Although the cant and wrought materials have similar, but not

identical compositions with regard to their chromium and nickel contents, metallurgicallythey have dierent structures. Where the wrought materials are ully austenitic,castings will contain some errite or more terminologically correct, ¶ (delta) errite, inthe basic austenitic matrix. The errite is necessary to permit welding to the castings,to avoid shrinkage cracking during cooling rom the casting temperature. It also actsas nucleation sites or the precipitation o chromium carbides which will invariably bepresent, as it will not always be possible to solution anneal the cast components.The nominal errite level is usually 5% to 12%. Below 5%, cracking problems may beexperienced. Above 12%, the errite tends to orm a continuous network rather than

remain as isolated pools.Because the crystallographic structures o the errite and austenite dier (austenitebeing a ace center cubic and the errite being a body center cubic), the errite has –thermodynamically speaking – a higher ree energy which renders it more susceptibleto attack, particularly in low pH chloride containing environments such as tomatoketchup and glucose syrups. Although theproblem is not so severe when the erriteoccurs as isolated pools, when present asa continuous network, propagation o the

corrosion occurs along the errite with theaustenite phase being relatively unaected– Figure 19. Because the products ocorrosion are not leached out rom thecorrosion site and are more voluminousthan the metal, corroded castings requentlyassume a blistered or pockmarkedappearance. The common environments

24

Fig. 19: Photomicrograph o a section o cast 316(CF8M) showing preerential attack o theerrite phase.



encountered in the industry whichproduce preerential errite attack are

the same as those causing stresscorrosion cracking. Thereore, this ormo damage is requently also present –Figure 20. Depending on the methodused to make the casting, the surace othe castings can be chemically modiied,which reduces the corrosion resistance.Small components are usually castby either the shell molding process or

produced as investment castings. In theshell molding process, the sand ormingthe mold is bonded together with anorganic resin which carbonizes whenthe hot metal is poured. This results inthe metal adjacent to the mold havingan enhanced carbon level with theormation o intergranular carbideprecipitates. Hence, there is a

susceptibility to intergranular attack andother orms such as crevice, pitting, andstress corrosion cracking. Methods oovercoming this include solutionannealing or machining o thecarburized skin o metal.With investment castings the mold ismade o zircon sand (zirconium silicate),and ired at a high temperature to

remove all traces o organic materialand wax, which is used as a core inthe mold making process. They do not,thereore, have this carburized layer andoer a much lower resistance to suracecorrosion.

3.2.8.2 free machining stainless steelsStainless steels are notoriously diicult to machine, especially turning, not so much

because they are hard, but because the swar tends to orm as continuous lengthswhich clog the machine and weld to the tip o the machine tools. One method oovercoming this is to incorporate a small amount, typically 0.2%, o sulphur or seleniumin the alloy. These elements react with the manganese to orm manganese sulphideor selenide. These are insoluble in the steel and orm as discreet pools in castingsor as elongated, continuous stringers in, say, wrought bar. The eect o the sulphideinclusions is to cause the material to orm chips, rather than long strings o swar when

25

Fig. 20: Corroded pump impeller (above) and valve body(below) caused by tomato ketchup. Note the pock-likecorrosion sites.



being machined. Both manganesesulphide and selenide have virtually zero

corrosion resistance to dilute mineralacids or other corrosive media. Thus,the ree cutting variants have a muchlower corrosion resistance than theirdesignation would imply. Indeed, somebelieve that the addition o sulphurto a type 316 material will oset thebeneicial eect o the alloying additiono molybdenum. As stated, the sulphide

inclusions will occur in castings asdiscrete pools. Thereore, there will notbe a continuous corrosion path. However,one o the products o corrosion in acidicmedia will be hydrogen sulphide, whichhas a proound eect on the corrosivityo even dilute mineral acids, causingattack o the austenitic matrix.In the case o wrought materials, in

particular bat stock, the sulphideinclusions are present as semi-continuous stringers and can suer so-called end-grainattack in mildly corrosive media.Stainless steel nuts and bolts, which are produced on automatic thread cuttingmachines, are invariably made rom ree-cutting materials. Figure 21 illustrates thedierence in corrosion resistance o a bolt made rom this and a non-reeing machining316. Both bolts were exposed to the same mildly acidic environment. When speciyingmaterials o construction, this dierence must be recognized. Any componentturned rom bar stock which is likely to come into contact with potentially corrosive

environments should always be speciied in 316 and not in the ree machining, sulphurcontaining variant.

4. CORROSION OF SPECIFIC ENVIRONMENTSFrom a corrosion standpoint, the environments likely to be encountered in the oodindustry which may cause premature equipment ailure may be classiied undermain headings:

Non-corrosive Alkaline detergentsMildly corrosive Acidic detergents

Highly corrosive Sanitizing agentsService luids

4.1 Non-corrosive foodstuffs

In general terms, natural oodstus such as milk, cream, natural ruit juice, and wholeegg do not cause corrosion problems with 304 or 316 stainless steels. Preparedoodstus to which there is no added salt such as yogurt, beer, ice cream, wine,spirits, and coee also all within this classiication. For general storage vessels,

26

Fig. 21: Free machining (let) and non-reemachining (right) bolts ater immersionin a mildly acidic environment.



pipelines, pumps, ittings or valves, grade 304 is perectly satisactory. However,or plate heat exchangers which are highly creviced and thereore prone to crevice

corrosion, grade 316 is requently employed. This oers a higher degree o protectionagainst some o the more acidic products such as lemon juice which may containsmall quantities o salt and also provides a higher level o integrity against corrosion byservice liquids and sanitizing agents.It is quite common to use sulphur dioxide or sodium bisulphate or the preservationo ruit juices and gelatin solutions. In such cases, storage vessels always shouldbe constructed rom 316. Although the sulphur dioxide is non-corrosive at ambienttemperature in the liquid phase, as a gas contained in signiicant quantities within thehead space in a storage tank, it tends to dissolve in water droplets on the tank wall.

In the presence o air, the sulphurous acid that orms is oxidized to suluric acid at aconcentration high enough to cause corrosion o 304 but not o 316.

4.2 Mildly corrosive foodstuffsThis category o oodstus covers products containing relatively low levels o salt andwhere pH values are below seven. Examples include glucose/ructose syrups andgelatin, the production o which may involve the use o hydrochloric acid. For storagevessels, pipelines, ittings, and pumps, grade 316 has established a good track record.Boiling pans in this grade o steel are perect or long and satisactory service. The

corrosion hazards increase in processing operations involving high temperatures andwhere the coniguration o the equipment is such as to contain crevices, especiallywhen the product contains dissolved oxygen. For example, multi-stage evaporatorsoperating on glucose syrup will usually have the irst stage, where temperaturesmay approach 100°C (212°F), constructed in a super stainless steel such as 904L.Subsequent eects where temperatures are lower and where the product has beendeoxygenated may be abricated in grade 316 stainless steel.As previously indicated, it is common practice to use sulphur dioxide as a preservativein dilute gelatin solutions during storage prior to evaporation. In some cases, excess

hydrogen peroxide will be added to neutralize the sulphur dioxide immediately beoreconcentration. This can give a catastrophic eect on the 300 series stainless steels andon even more highly alloyed metals such as 904L, due to the combined eect o thechlorides present in combination with the excess hydrogen peroxide. Because o this,it is a more acceptable practice to make the peroxide addition ater, rather thanbeore, evaporation.Gelatin or pharmaceutical end use is subject to UHT treatment to ensure sterility.This will involve heating the gelatin solutions to 135°C (285°F) and holding at thattemperature or a short period o time. Plate heat exchangers are extensively used or

this duty. Although plates made rom 316 stainless steel give a reasonable lie otypically two to three years, a corrosion-resistant alloy with an enhanced level omolybdenum is preerred.

4.3 Highly corrosive foodstuffsThe list o oodstus alling in this category is almost endless – gravies, ketchups,pickles, salad dressings, butter, margarine – anything to which salt has been added at

27



the 1-3% level or even higher. Also within this category must be included cheesesalting brine and other brines used in the preservation o oodstus which undergo

pasteurization to minimize bacterial growth on ood residues remaining in the brine.Although these brines are usually too strong to support the growth o commonorganisms, salt resistant strains (halophiles) are the major problem.Low pH products containing acetic acid are particularly aggressive rom a corrosionstandpoint, but selection o materials or handling these products very much dependson the duty involved.When trying to deine the corrosion risk to a piece o equipment handling potentialcorrodents, several actors come into play. While temperature, oxygen content, chloridecontent, and pH are the obvious ones, less obvious and equally important is contact

time. All three main orms o corrosion induced in stainless steels (crevice, pitting, andstress corrosion cracking) have an induction period beore the onset o corrosion.This can vary rom a ew hours to several months, depending on the other operativeactors. In a hypothetical situation where stainless steel is exposed to a potentiallycorrosive environment, removal o the steel and removal o the corrodents will stop theinduction and the status quo is established. On repeating the exposure, the inductionperiod is the same. In other words, the individual periods the steel spends in contactwith the corrodent are not cumulative and each period must be taken in isolation.When the contact period is short, temperatures are low and a rigorous cleaning regime

is implemented at the end o each processing period – 316 stainless steel will giveexcellent service. However, where temperatures are high and contact periods are long,the corrosion process may be initiated. This is especially common in crevices such asthe interplate contact points on a plate heat exchanger, where albeit at a microscopiclevel, corrodents and corrosion products are trapped in pits or cracks. Geometricactors may prevent the complete removal o this debris during cleaning. Under suchcircumstances, the corrosion process will be ongoing.Due to the perishable nature o oodstus, storage is rarely or prolonged periods orat high temperatures and regular, thorough cleaning tends to be the norm. The one

exception to this is buer storage vessels or holding “sel preserving” ketchup andsauces. For such duties, an alloy such a 904L, Avesta 254SMO, or even Inconel 625may be required.While all the oregoing applies to general equipment, the one exception is plate heatexchangers. Their highly creviced coniguration and the high temperatures employedmake them particularly susceptible. Plates made rom grade 316 have a poor trackrecord on these types o duties. Even the more highly alloyed materials do not oercomplete immunity. The only reasonably priced material which is inding increasedusage in certain areas o ood processing is titanium.

The act that butter and margarine have been included in this group o corrodentsrequires comment. Both these oodstus are emulsions containing typically 16% waterand 2% salt. A act not oten appreciated is that the salt is dissolved in the water phase,being insoluble in the oil. From a corrosion standpoint, thereore, the margarine orbutter may be regarded as a suspension o 12% salt solution, and as such, is verycorrosive to 316 stainless steel at the higher processing temperatures. The onlymitigating eature which partly osets their corrosivity is the act that the aqueous saltyphase is dispersed in an oil rather than the reverse, and the oil does tend to

28



preerentially wet the steel surace and provide some degree o protection. However,the pasteurizing heat exchanger in margarine rework systems invariably has titanium

plates as the lie o 316 stainless steel is limited and has been known to be as little assix weeks.

4.4 Corrosion by service fluids

4.4.1 SteamBeing a vapor and ree rom dissolved salts, steam is not corrosive to stainless steels.Although sometimes contaminated with traces o rust rom carbon steel steam lines,in the author’s experience no case o corrosion due to industrial boiler steam ever has

been encountered.

4.4.2 WaterThe quality and dissolved solids content o water supplies varies tremendously withthe aggressive ionic species, chloride ions, being present at levels varying betweenzero, as ound in the lakeland area o England, to several hundred parts per million,as encountered in coastal regions o Holland. It is also normal practice to chlorinatepotable water supplies to kill pathogenic bacteria with the amount added dependenton other actors such as the amount o organic matter present. However, most water

supply authorities aim to provide water with a residual chlorine content o 0.2 ppm atthe point o use. Well waters also vary in composition depending on the geographicallocation, especially in coastal regions where the chloride content can luctuate with therise and all o the tide.What constitutes a “good” water? From a general viewpoint, the important actor ishardness, either temporary hardness caused by calcium and magnesium bicarbonateswhich can be removed by boiling, or permanent hardness caused by calcium sulphatewhich can be removed by chemical treatment. While hardness is a actor, chloridecontent and pH probably are the most important rom a corrosion standpoint. What

can be classiied as a non-corrosive water supply rom a stainless steel equipmentuser? Unortunately, there is no hard and ast rule which will determine whether or notcorrosion o equipment will occur. As repeatedly stated throughout this article, manyactors come into play. The type o equipment, temperatures, contact times, etc., all playa role in the overall corrosion process. Again, as stated beore, the most critical itemso equipment are those with inherent crevices – evaporators and plate heat exchangers,among others. Deining conditions o use or this type o equipment will be a regulatoryactor. Even then, it is virtually impossible to deine the composition o a “sae” water,but as a general guideline, water with less than 100 ppm chloride is unlikely to initiate

crevice corrosion o type 316 stainless steel, while a maximum level o 50 ppm shouldbe used with type 304.Cooling tower water systems are requently overlooked as a potential source ocorrodents. It must be appreciated that a cooling tower is an evaporator, and althoughthe supply o makeup water may contain only 25 ppm chloride, over a period ooperation this can increase by a actor o ten unless there is a routine bleed on thepond.

29



Water scale deposits ormed on heat transer suraces should always be removedas part o the routine maintenance schedule. Water scale deposits can accumulate

chloride and other soluble salts which tend to concentrate, producing higher levels incontact with the metal than indicated by the water composition. Furthermore, waterscale ormed on a stainless steel surace provides an ideal base or the onset o crevicecorrosion.As previously stated, potable water supplies usually have a residual ree chlorinecontent o 0.2 ppm. Where installations have their own private wells, chlorination isundertaken on site. In general terms, the levels employed by the local water authoritiesshould be ollowed and over-chlorination avoided. Levels in excess o 2 ppm couldinitiate crevice corrosion.

4.4.3 Cooling brinesDepending on the industry, these can be anything rom glycol solutions, sodium nitrate/carbonate or calcium chloride. It is the latter which is used as a 25% solution. Thiscan give rise to corrosion o stainless steel unless maintained in the ideal condition,especially when employed in the inal chilling section o plate heat exchangers or milkand beer processing. However, by observing certain precautions, damage can beavoided.The corrosion o stainless steels by brine can best be represented as shown in Figure

22. An exponential rise in corrosion rate with reducing pH occurs in the pH range o6-4 and corresponds with the change in mode o attack, i.e., orm pitting to general

30

80

70

60

50

40

30

20

10

010 8 6 4 2

pH

N U M B E R O F P I T S / S Q D M

PITTINGATTACK

PITTING/

GENERALATTACK

GENERALATTACK

40 50 60 70 80 90 100

TEMPERATURE ºC

0

4

8

12

16

20

24

W E I G H T L O S S - M G / D M / D A Y

28

32

Fig. 22: The eect o pH and temperature on the pitting o stainless steel by brine.



corrosion. It will be seen that, ideally, the pH o the solution should be maintained in theregion 14-11. However, calcium chloride brine undergoes decomposition at pH values

higher than 10.6:CaCl2 + 2NaOH +2NaClWhen scale deposition occurs heat transer suraces become ouled with calciumhydroxide scale. Furthermore, the scale which orms traps quantities o chloride saltswhich cannot be eectively removed and remain in contact with the equipment duringshutdown periods. This is particularly important in equipment such as plate heatexchangers which are subjected to cleaning, and possibly hot water sterilization cyclesat temperatures o 80°C (176°F) or higher.The other aspect, non-aeration, is equally important. Air contains small quantities o

carbon dioxide which orm a slightly acidic solution when dissolved in water. This hasthe eect o neutralizing the buering action o any alkaline component in the brine:i.e., 2NaOH + CO2 > NaCO3 + H2O

Na2CO3 + CaCl2 > 2NaCl + CaCo3or Ca(OH)2 + CO2 > CaCO3 + H2OThereore, the pH o the brine decreases and assumes a value o about pH 6.5, whichis the region where pitting incidence is highest. Furthermore, scale deposits o calciumcarbonate are laid down on heat transer suraces creating the problems reerred toabove.

The precautions to be observed when using brine circuits are:• Ensure correct pH control and maintain in the pH range of 9.5-10.• Eliminate aeration. In particular, make certain the brine return discharge line is below

the surace in the storage tank during running and that the method o eeding brinerom the tank does not cause vortexing with resultant air entrainment. Baudelotevaporators cause aeration o the chilling liquor and should never be used on brinecircuits.

• When cleaning and sterilizing the brine section of a pasteurizer, flush out allbrine residues until the rinse water is ree o chloride. As an added precaution, it is

advisable to orm a closed circuit and circulate a 1/4% - 1/2% caustic soda or sodiummetasilicate solution to ensure that any brine residue is rendered alkaline.

• In plate heat exchangers and similar equipment, make sure that stainless steel brinesection components remain ree o scale.

• When shutting down the plant after a cleaning run, it is advisable to leave the sectionull with a dilute (1/2%) caustic solution. Beore startup, this should be drained andresidues lushed out prior to re-introducing brine.

When operating conditions prevailing in a plant do not permit such a disciplinedcleaning, operating, and shutdown procedure, only two materials can be considered or

the brine section o a plate heat exchanger. These are Hastelloy C-276 or titanium. Thecorrosion resistance o both o these materials is such that cleaning and sanitizing othe product side o the heat exchanger can be carried out without removing the brine.Although both are more expensive than stainless steels, especially Hastelloy C-276, thelexibility o plant operation (which their use permits) could oset their premium prices.

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4.5 Alkaline detergentsSupplied to ood processing plants either as bulk shipments o separate chemicals or

as careully preormulated mixtures, the composition o alkaline detergent ormulationscan vary widely in accordance with individual preerence or the cleaning job to be done.The detergents, however, generally include some or all o the ollowing compounds:

sodium hydroxidesodium polyphosphatesodium metasilicatesodium carbonate

Additionally, it is common to ind that a selection o sequestering agents such asE.D.T.A. and any o the many available wetting agents also may be present in the

ormulations. None o these compounds are corrosive to stainless steels at theconcentrations and temperatures used by the ood industry or cleaning. 316 stainlesssteel is unaected by concentrations o sodium hydroxide, as high as 20%, attemperatures up to 160°C (320°F). They can, thereore, be used with impunity at theirusual maximum concentration o 5%, even in UHT operation where temperatures canrise to 140°C (284°F).Companies have reported that some o these preormulated alkaline detergents causediscoloration o the equipment.The discoloration starts as a golden yellow, darkening to blue through mauve and

eventually black. It has been established that this discoloration is caused by theE.D.T.A. sequestering agent which complexes with traces o iron in the water. It thendecomposes under certain conditions o pH and temperature to orm an extremely ineilm o hydrated iron oxide, the coloration being intererence colors which darken as theilm thickness increases. Although the ilm is not aesthetically pleasing, it is in no waydeleterious and removing it by conventional cleaning agents is virtually impossible.Some alkaline detergents are compounded with chlorine release agents such as sodiumhypochlorite, salts o di- or trichlorocyanuric acid which orm a solution containing200-300 ppm available chlorine at their usage strength. Although the high alkalinity

reduces the corrosivity o these additives, generally speaking, they should not beemployed on a regular basis at temperatures exceeding 70°C (160°F).

4.6 Acidic detergentsAlkaline detergents will not remove the inorganic salts such as milkstone and beerstonedeposits requently ound in pasteurizers. For this, an acidic detergent is required andselection must be made with regard to their interaction with the metal. Suluric andhydrochloric acids will cause general corrosion o stainless steels. Although it could beargued that suluric acid can be employed under strictly controlled conditions because

stainless steels, especially grade 316, have a very low corrosion rate, its use couldresult in a deterioration o surace inish. This, in corrosion terms, is an extremely lowrate but rom an aesthetic viewpoint, is undesirable.Acids such as phosphoric, nitric and citric, when used at any concentration likely to beemployed in a plant cleaning operation, have no eect on stainless steels and can beused with impunity. Three cautionary notes are worthy o mention:• It is always preferable to use alkaline cleaning before the acid cycle to minimize the

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risk o interacting the acid with any chloride salt, and thereore, minimize the ormationo hydrochloric acid.

• It is inadvisable to introduce an acid into a UHT sterilizing plant, when it is at fulloperating temperature (140°C/285°F), as part o a “clean-on-the-run” regime.

• Nitric acid, being a strong oxidizing agent, will attack certain types of rubber used asgaskets and seals. As a general guideline concentrations should not exceed 1%and a temperature o 65°C (150°F), although at lower concentrations the upperrecommended temperature is 90°C (195°F).

Another acid which is inding an increased use in the ood industry or removing waterscale and other acid soluble scales is sulamic acid. Freshly prepared solutions o upto 5% concentration are relatively innocuous to stainless steels, but problems may

arise when CIP systems incorporating recovery o detergents and acids are employed.Sulamic acid will undergo hydrolysis at elevated temperatures to produce ammoniumhydrogen sulate.NH2SO2OH + H2O > NH4HSO4This behaves in much the same way as suluric acid. In situations where the use o thisacid is contemplated, prolonged storage o dilute solutions at elevated temperatures isinadvisable, although at room temperature the hydrolysis is at a low rate.

4.7 Sanitizing agents

Terminology or the process o killing pathogenic bacteria varies rom country to country.In Europe, disinection is preerred; in America, sanitizing. Regardless, the term shouldnot be conused with sterilization which is the process o rendering equipment ree romall live ood spoilage organisms including yeasts, mold, thermophilic bacteria, and mostimportantly, spores. Sterilization with chemicals is not considered to be easible andthe only recommended procedure involves the circulation o pressurized hot water at atemperature o not less than 140°C (285°F).For sanitization, while hot water (or steam) is preerred, chemical sanitizers areextensively used. These include non-corrosive compounds such as quaternary

ammonium salts, anionic compounds, aldehydes, anphoterics, and potentially corrosivegroups o compounds which rely on the release o halogens or their eicacy. By ar,the most popular sanitizer is sodium hypochlorite (chloros), and this is probably the onematerial that has caused more corrosion in ood plants than any other cleaning agent.For a detailed explanation o the corrosion mechanism, reer to an article by Boulton andSorenson (Re. 17) which describes a study o the corrosion o 304 and 316 stainlesssteels by sodium hypochlorite solutions. It is important, i corrosion is to be avoided, thatthe conditions under which it is used are strictly controlled. For equipment manuacturedrom grade 316 stainless steel, the recommended conditions are:

• Maximum concentration – 150 ppm available chlorine.• Maximum contact time – 20 minutes.• Maximum temperature – room temperature, which is well in excess of the minimum

conditions established by Tastayre and Holley, to kill pseudomonas aeruginosa(Re. 18).

In addition, several other precautions must be observed:• Before introducing hypochlorite, equipment should be thoroughly cleaned and free of

33



scale deposits. Organic residues reduce the bactericidal eiciency o the disinectantand oer an artiicial crevice in which stagnant pools o hypochlorite can accumulate.

• It is imperative that acidic residues be removed by adequate rinsing beforeintroducing hyperchlorite solutions. Acid solutions will react with hypochlorite torelease elementary chlorine, which is extremely corrosive to all stainless steels.

• The equipment must be cooled to room temperature before introducinghypochlorite. In detergent cleaning runs, equipment temperature is raised to 80/85°C(176/185°F) and unless it is cooled during the rinsing cycle, a substantial increase intemperature o the disinectant can occur. An important point, requently overlooked,is that a leaking steam valve can cause a rise in the temperature o equipment eventhough it is shut o.

• After sanitizing, the solution should be drained and the system flushed with water of anacceptable bacteriological standard. This normally is done by using a high rinse rate,preerably greater than that used in the processing run.

While these comments relate speciically to the sanitizing o plate heat exchangers,similar precautions must be taken with other creviced equipment. Examples includemanually operated valves which should be slackened and the plug raised to permitlushing o the seating surace. Pipeline gaskets also should be requently checked tomake sure that they are in good condition and not excessively hardened. Otherwise,they will ail to orm a crevice-ree seal over their entire diameter. Where it is not

possible to completely remove hypochlorite residues, such as in absorbent glandpacking materials, hot water is preerred.All the oregoing speciically relate to sodium hypochlorite solutions. Other sanitizingagents which rely on halogen release, such as di- and trichlorocyanuric acid, shouldalso be used under strictly controlled conditions.Iodohpors also are used or sanitizing equipment. These are solutions o iodine in non-ionic detergents and contain an acid, such as phosphoric, to adjust the pH into therange at which they exhibit bactericidal eicacy. This group o sanitizers is employedwhere hot cleaning is not necessary or on lightly soiled suraces such as milk road

tankers, arm tanks, etc. Extreme cautions should be exercised with this group. Althoughused at low concentrations (50 ppm), prolonged contact with stainless steel cancause pitting and crevice corrosion. Furthermore, in storage vessels which have beenpartially illed with iodophor solutions and allowed to stand overnight, pitting corrosionin the head space has been observed due to iodine vaporizing rom the solution andcondensing as pure crystals on the tank wall above the liquid line. Another actor is thatiodine can be absorbed by some rubbers. During subsequent processing operations atelevated temperatures, the iodine is released in the orm o organic iodine compounds,especially into atty oods, which can cause an antiseptic taint. The author knows o one

dairy which used an iodophor solution to sanitize a plate pasteurizer to kill an inectiono a heat resistant spore-orming organism. The ollowing day, there were over 2,000complaints o tainted milk. CIP cleaning cycles did not remove the antiseptic smell romthe rubber seals and complete replacement with new seals was the only method oresolving the problem.Another sanitizing agent which is increasing in popularity, especially in the brewingindustry due to its eicacy against yeasts, is peracetic acid. It will not cause corrosiono 304 and 316 stainless steels and the only precautionary measure to be taken is to

34



use a good-quality water containing less than 50 ppm o chloride ions or making up thesolutions to their usage concentration. Due to the strongly oxidizing nature o some

types o peracetic acid solutions, deterioration o some types o rubber may occur. Arecent survey, undertaken by the IDF, o the use o peracetic acid in the dairy industry(Re. 19) ound ew corrosion problems were reported. The general consensus wasthat it permitted greater lexibility in the conditions o use, compared with sodiumhypochlorite, without running the risk o damage to equipment.For comprehensive inormation on the cleaning o ood processing equipment, albeitprimarily written or the dairy industry, reer to the British Standards Institutepublication BS 5305 (Re. 20).

5. CORROSION BY INSULATING MATERIALSEnergy conservation is now widely practiced by all branches o industry, the oodindustry being no exception. For example, in the brewing industry, wort rom the wortboilers is cooled to ermentation, and the hot water generated in the process is storedin insulated vessels (hot liquor tanks) or making up the next batch o wort. An area ocorrosion science which is receiving increased attention is the subject o corrosioninitiated in stainless steels by insulating materials. At temperatures in excess o 60°C(140°F) these can act as a source o chlorides which will induce stress corrosioncracking and pitting corrosion o austenitic stainless steels.

Among the insulating materials which have been used or tanks and pipework are:Foamed Plastics – polyurethane

polyisocyanuratephenic resins, etc.

Cellular and oamed glassMineral iber – glass wool, rock woolCalcium silicateMagnesiaCork

All insulating materials contain chlorides to a lesser (10 ppm) or greater (1.5%) extent.The mineral-based insulants, such as asbestos, may contain them as naturally occurringwater soluble salts such as sodium or calcium chloride. The organic oams, on the otherhand, may contain hydrosoluble organo-chlorine materials used as phosphates, orchlorine containing materials present as impurities. Insulation manuacturers arebecoming increasingly aware o the potential risk o chlorides in contact with stainlesssteels and are making serious eorts to market a range o materials which areessentially chloride ree.Even though many o the insulating materials contain high levels o chloride, in

isolation they are not corrosive.Corrosion, being an electrochemical process, involves ionic species. In the absence oa solvent (water) the chloride or salts present in the insulation cannot undergo ionizationto give chloride ions. Thereore, they essentially are non-corrosive. Similarly, whereorgano-chlorine compounds are present, water is necessary or hydrolysis to occur withthe ormation o hydrochloric acid or other ionizable chloride compounds.The main problem, thereore, is not so much the insulant but the interaction o

35



the insulant with potential contaminants to release corrosive species. Under idealconditions, i the insulating material could be maintained perectly dry, then the

chloride content would not be a critical actor in the material speciication process.Unortunately, it must be acknowledged that even in the perectly regulated installation,this ideal is rarely (i ever) achieved. Thereore, thought must be given to recommendedguidelines.Any material which is capable o absorbing water must be regarded as a potentialsource o chloride. Although the chloride content o the insulant may be extremelylow (25 ppm), under extractive conditions when the insulation becomes wet andconcentration eects come into play, even this material may cause the initiation ostress corrosion cracking. The chloride content o the contaminating water cannot be

ignored. Even a “good quality” water with a low chloride content (say 30 ppm), i beingcontinuously absorbed by the insulant, orms a potential source o chlorides. Throughevaporation, it can reach very signiicant levels and initiate the corrosion mechanism.The more absorptive the insulant, the greater the risk, and materials such as calciumsilicate and certain types o oams must be regarded as least desirable. It is interestingto note that one o the least absorptive insulants rom those listed is cork. In theexperience o the author, there has been no reported case o this insulant causingproblems with stress corrosion cracking o stainless steels. O course, it could beargued that the bitumen used as an adhesive to stick the cork to the vessel walls has

to be applied so thickly that the bitumen orms an impermeable barrier preventingcontact o contaminated water with the stainless steel substrate. Irrespective o theprotection mechanism, the net eect is that cork has an extremely good “track record.”Unortunately, due to the cost, cork is now rarely used.No hard and ast rule can be applied or speciying the acceptable, maximum,tolerable, chloride content o an insulating material. Any speciication must take intoaccount what is commercially available, as well as all the other actors such as price,lammability, ease o application, etc. To speciy zero chloride is obviously impracticaland even a igure o 10 ppm may be diicult to achieve in commercially available

products. As a general compromise, a 25 ppm maximum is considered to be technicallyand commercially easible while minimizing the potential source level o chloridecorrodent.The primary unction o the insulant is to provide a thermal barrier between theoutside o the vessel and the environment. It does not provide a vapor or moisturebarrier and provision o such protection must be regarded as equally important in theinsulating process. As previously mentioned, the use o bitumen as an adhesive orcork insulation must provide an extremely good water barrier, although there are awide range o products marketed speciically or this purpose. These include specially

developed paints o undeined composition and zinc ree silicone alkyd paints, as wellas silicone lacquer. These silicone-based products are particularly appropriate becauseo their inherent, low-water permeability and also their stability at elevated temperatures.Aluminum oil has also been successully used as a water/vapor barrier between thestainless steel and the insulant. There is little doubt that the oil provides an extremelygood barrier but at laps between the sheets o oil, ingression o water can occurunless sealing is complete, the achievement o which is most unlikely. Furthermore, it islikely that there will be tears in the oil, providing yet another ingression path or water.

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However, it is believed that aluminum oil has a role additional to that o a barrier. Froman electrochemical aspect, being “anodic” to stainless steel, it will provide galvanic

protection o the steel in areas where there is a “holiday” in the oil, thus inhibitingcorrosion mechanisms.When insulation material becomes wet, the insulating eiciency shows a dramatic allo. In act, the thermal conductivity o wet insulation will approach that o the wettingmedium. It is ironic that the thermal conductivity o water is among the highest knownor liquids. It is imperative that rom the standpoint o preventing moisture ingressionto minimize corrosion risks and maintain insulation eiciency, the insulation is externallyprotected rom rain and water. There is a variety o materials available or this suchas aluminum sheeting, plastic coated mild steel, spray applied polyurethane coatings,

etc. Much o the value o the protection will be lost unless particular attention is takento maintaining a weather-tight seal at the overlaps in individual sheets o cladding.There are many semi-lexible caulking agents which can be employed or this. Onewhich is particularly eective is the RTV silicone rubber which exhibits extremely goodweather resistance and long-term reliability. Nevertheless, maintenance is required,especially around areas o discontinuity such as langed connections, manway doors,etc. Operators are beginning to realize that routine maintenance work on insulation is asimportant as that on all other items o plant and equipment.Insulation o a vessel or pipeline is a composite activity with many interactive

parameters. It is impossible to set speciication rules, as each case must be viewed inthe light o requirements. Thereore, these notes must be regarded as guidelines ratherthan dogma. For urther inormation, an excellent publication by the American Society orTesting Materials (Re. 21) is essential reading.

6. CORROSION OF RUBBERSMany involved in the ield o corrosion would not regard the deterioration o rubberas a corrosion process. The author’s opinion diers rom this viewpoint as rubberundergoes deterioration by interaction with its environment.

6.1 GeneralRubber and rubber components orm an essential part o ood processingequipment – joint rings on pipelines, and gaskets on heat exchangers and plateevaporators. Although natural rubber was the irst material to be used or manuacturingthese components, today they are made almost exclusively rom one o the syntheticrubbers listed in Table 5.Unlike metals and alloys which have a strictly deined composition, the constituentsused in the ormulation o rubbers are rarely stipulated. More oten, they will relect the

views and idiosyncrasies o the ormulator on how to achieve the desired end product.The important constituents o a rubber are:• Basic polymerLargely determines the general chemical properties o the inished product.• Reinforcing fillersThese are added to improve the mechanical properties and will invariably be one o the

37



grades o carbon black – or i a white rubber is required, mineral illers such as clays orcalcium silicate.• Vulcanizing agentsThese cross link the basic polymer and impart rubber-like properties which aremaintained at elevated temperatures.• Anti-oxidantsTo stabilize the rubber against oxidative degradation, hardening or sotening, aterprolonged operating periods at elevated temperatures.• Processing aidsWhich acilitate the molding o the rubber.• PlasticizersTo modiy the mechanical properties.A complicating actor which has to be considered when ormulating rubber or oodcontact suraces is the acceptability (or non-acceptability) o the compoundingingredients. Some countries, notably Germany and the U.S.A., have drawn up lists opermitted ingredients (Re. 22, 23). Other countries regulate the amount o materialwhich can be extracted rom the inished article by various test media.Invoking these regulations may impose limits on the in-service perormance o a rubbercomponent which could be a compromise, exhibiting desirable properties inerior tothose achievable i it were or a non-ood application. For example, the resistanceto high-pressure steam o some rubbers can be enhanced by using lead oxide as aningredient. Obviously, such materials could not be contemplated or any ood contactapplication.

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Rubber Common Trade Names Basic Structure

Polychloroprene Neoprene Poly (2-chlor-1, 3 butadiene)Perbunan CButachlorNairit

SBR Buna S Co-polymer o styrene and(Styrene butadiene rubber) Pliolex Ind 1, 4 butadiene

Intol KryleneNitrile Buna N Co-polymer o acrylonitrile

Chemigum N Ind 1, 4 butadieneParacilPerbunanHycar

EPDM Nordel Co-polymer o ethylene and(Ethylene propylene Royalene proplylene with a third monomerdiene methylene) Vistalon such as ethylidene norbonene,

Dutral cyclopentadiene, etc.KeltanIntolan

Butyl GR-1 Co-polymer o isobutyleneBucar and isopreneSocabutyl

Silicone Silastic e.g., Poly dimethyl siloxaneSilastomer Poly methyl vinyl siloxaneSil-O-Flex

Fluoroelastomer Viton Co-polymer o hexaluoropropyleneTechnolon and vinylidene luorideFluorel

Table 5



6.2 Corrosion by rubberThe majority o rubbers and ormulating ingredients have no eect on stainless steelseven under conditions o high temperature and moisture. There are two notableexceptions, polychloroprene and chlorosulphonated polyetheylene. Both o thesecontain chlorine, which under the inluence o temperature and moisture,undergo hydrolysis to produce small quantities o hydrochloric acid. In contact withstainless steel, this represents a serious corrosion hazard causing the three main ormso attack.When speciying a rubber component, it is easy to avoid these two polymers but many

o the rubber adhesives are produced rom one o these polymers. That is the reasonwhy manuacturers o heat exchangers and other equipment which necessitates stickingthe rubber onto metal, speciy what type o adhesive should be used. Many DIYadhesives and contact adhesives are ormulated rom polychloroprene. It is common ora maintenance engineer to stick gaskets in a heat exchanger to get his/her supply orubber cement rom the local hardware shop. The results can be catastrophic as shownin Figure 23. Similarly, many sel-adhesive tapes use a polychloroprene-based adhesive,and direct contact o these steels should be avoided.

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Fig. 23: Catastrophic ailure o the gasket groove o a heat exchanger plate by stress corrosion cracking causedby the use o a polychloroprene-based adhesive.



6.3 Corrosion of rubber by environments

From the standpoint o ood processing, the environments likely to interact withrubber are classiied under the ollowing headings:• Foodstuffs containing no fat or a low level of fat, e.g., milk.• Fatty products: butter, cream, cooking oils, shortenings.• Alkaline detergents.• Acid detergents.• Sanitizing agents.Unlike the corrosion o metals which is associated with oxidation and loss o metal,rubber deterioration usually takes other orms. When a rubber is immersed in a liquid, it

absorbs that liquid or substances present in it to a greater or lesser degree.The amount o absorption determines whether the rubber is compatible with theenvironment. The absorption will be accompanied by changes in mass, volume,hardness and tensile strength. For example, immersing an oil-resistant rubber invegetable oil may produce a change in volume o only 2-3%, whereas a non-oil-resistant rubber may swell by 150% or more. Such a volumetric change will beaccompanied by a large reduction in the tensile strength and a high degree o sotening.

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Table 6: Perormance o rubbers in some environments ound in the ood and beverage industries.

Poly-Medium Natural chloro- Silicon

SBR Nitrile Rubber prene Butyl EPDM (7)

Products

Whole milk E E E E E E E

Beer, wines and spirits G-F E F E E E EFats, oils and cream F E F G P G E

Sauces E E F (8) E E (8) E E

Salad dressings F E F G P G E

Fruit drinks and juices (9) E E G E E E E

Cleaning Agents

Sodium hydroxide (1) G E G G E E E

Sodium carbonate (2) E E E E E E E

Sodium hypochlorite (3) G G G G E E E

Nitric acid (4) F F P F G G G

Phosphoric acid (5) E E E E E E E

Quaternary ammonium E E E E G G Gcompounds (6)

Notes:

(1) All strengths up to 5% at maximum operating temperatures o the rubber.

(2) Sodium carbonate and other detergent/additives, e.g., sodium phosphate, silicate.

(3) Sodium hypochlorite as used at normal sterilizing concentration – 150 mgl-1.

(4) Nitric acid as used at normal cleaning strength o1

/2

to 1%.(5) Up to 5% strength.

(6) Used as an aqueous 1% solution.

(7) Depending on the type o basic polymer.

(8) Depending on the at/oil content.

(9) The perormance o a rubber will be aected by the presence o essential oils.



Broadly speaking, a rubber should not exhibit a volumetric or weight change greaterthan 10%, nor a hardness change o more than ten degrees (International RubberHardness Degrees – IRHD or Shore A) to be classiied as compatible. Data presentedin Tables 6 and 7 indicate the compatibility o rubbers with some ood industryenvironments. But or more inormation, reer to one o the national or international testprocedures (Res. 24, 25, 26).

After a basic training in chemistry, author Dr. Colin Cowan undertook academic

and industrial research into the use of clay minerals as absorbents. Dr. Cowan, who is

currently retired after more than 30 years of service with APV, headed a British based

multidiscipline laboratory dealing with all aspects of materials technology. He also

operated a technical services laboratory in Tonawanda, NY with emphasis on the

specification of metallic and non-metallic materials for processing equipment.

41

Table 7: Suggested limits o concentration and temperature or peracetic acid in contact with rubbers.

Peracetic AcidConcentration Temp. Medium Butyl Fluoro

(active ingredient) °C Nitrile (resin cured) EPDM Silicon Elastomer

20 R R R R R60 R R R R R85 (?) R R R (?)

20 R R R R R60 N.R. R R N.R. R85 N.R. (?) R N.R. N.R.

20 R R R (?) R60 N.R. R R N.R. R85 N.R. N.R. R N.R. N.R.

20 — R R N.R. R60 N.R. R R N.R. R85 N.R. N.R. R N.R. N.R.

0.05%(500 mg 1-1)

0.10%(1 mg 1-1)

0.25%(2.5 mg 1-1)

0.5%(5 mg 1-1)

R – Little eect on the rubber (?) – Possibly some degradation

N.R. – Not recommended as signiicant degradation may occur



REFERENCES

1. Warren, N., 1980 Metal Corrosion in Boats. London Stanord Maritime2. Sedrick, A.J., Corrosion o Stainless Steels, Wiley, NY, 19793. The British Aluminum Co., Ltd., Aluminum in the Chemical and Food Industries, 19514. Ferry, G., New Scientist, 27 February 1986, 235. Demmett, G.A., From Little Acorns, A History o APV Company, Hutchison Benham6. Cowan, C.T., Corrosion Engineering with New Materials, The Brewer 163-168, 19857. Fontana, M.G., Corrosion Engineering, McGraw-Hill, NY 19868. Shrier, L.L., Corrosion, Newnes-Butterworth, London 19769. Oldield, J.W., and Todd, B., Trans. Inst. Mar. Eng. (C), 1984, Con. 1,139

10. Oldield, J.W., Test Techniques or Pitting and Crevice Corrosion Resistance oStainless Steels and Nickel Based Alloys in Chloride-containing Environments, NiDiTechnical Series 10 016, 1987

11. Evans, U.R., Corrosion, 1981 7,23812. A.S.T.M. G48-76, Standard Methods or Pitting and Crevice Corrosion Resistance

o Stainless Steels and Related Alloys by use o Ferric Chloride Solution, AmericanSoc. For Testing Materials, Philadelphia, PA

13. Brigham, R.J., Materials Perorm, Nov. 1974, 13, 2914. Greene, N.D., and Fontana, M.G., Corrosion, 1959, 15, 25t

15. Woelul, M. and Mulhall, R., Metal Progress, 57-59, Sept. 198216. Copson, M.R., Eect o Composition on SCC o Some Alloys Containing Nickel, In

Rhodin, T. (ed.). Physical Metallurgy o Stress Corrosion Fracture, Interscience Pub.,Inc. NY 1989

17. Boulton, L.H., and Sorenson, M.M., N.Z. Journal o Dairy Science & Technology,1988, 23, 37-49

18. Tastayre, G.M., and Holley, R.A. (1986) Publication 1806/B. Agriculture Canada,Ottawa

19. I.D.F. Bulletin 236, Corrosion by Peracetic Acid Solutions, International Dairy

Federation, 41 Square Vergote, 1040 Brussels, Belgium20. BS 5305 Code o practice or cleaning and disinecting o plant and equipment

used in the dairying industry. British Standards Institute, London21. A.S.T.M. Special Technical Publication 880, Corrosion o Metals under Thermal

Insulation, American Society or Testing Materials, Philadelphia, PA22. Sonderdruck aus Bundesgesundheitsblattm 22, 1, 79, W. Germany23. USA Code o Federal Regulations, Title 21, Food and Drugs Section 177.2600,

Rubber Articles Intended or Repeated Use24. BS 903 Part A.16, Resistance o Vulcanized Rubbers to Liqiuds, Br. Stds. Inst.,

London25. A.S.T.M. D. 471 Testing o Rubbers and Elastomers. Determination o Resistance

to Liquids, Vapors, and Gases. American Society or Testing Materials, Philadelphia,PA

26. I.S.O. 1817, Vulcanized Rubbers – Resistance to Liquids – Methods o Test

42



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