Chapter - 3

Group 1Identifiable by

visual inspection

Group 2May require

supplementary means of

examination

Group 3Verification is

Usually required by microscopy

(optical, electron microscopy etc.)

• Uniform Corrosion• Pitting Corrosion• Crevice corrosion

Crevice model Pack rust

• Galvanic corrosion• Lamellar corrosion

• Erosion corrosion• Cavitation• Fretting corrosion• Intergranular corrosion• Exfoliation• Dealloying

• Environmental cracking

Stress Corrosion Cracking (SCC) Corrosion fatigueHydrogen embrittlement

• Corrosion damage can take many shapes and forms that are often related to specific alloy/environment/operation conditions. The several forms of corrosion may be divided into three groups:

Classification of Corrosion

General or Uniform Attack

Localized Corrosion

Velocity Induced

Corrosion

Mechanically Assisted

Corrosion

• Pitting corrosion• Crevice corrosion• Galvanic corrosion• Intergranular corrosion• Dealloying• Hydrogen induced cracking (HIC)• Hydrogen embrittlement (HE)

• Erosion-corrosion• Cavitation• Impingement

• Stress corrosion cracking• Corrosion fatigue• Fretting corrosion

Uniform Corrosion

Galvanic Corrosion

Pitting Corrosion

Crevice Corrosion

FiliformCorrosion

Fretting Corrosion

Corrosion Fatigue

Stress Corrosion Cracking

Intergranular Corrosion

Erosion Corrosion Dealloying Hydrogen

Damage

Microbial Corrosion

Corrosion in Concrete

Concentration Cell Corrosion

This is also called general corrosion. The surface effect produced by most direct chemical attacks (e.g., as by an acid) is a uniform etching of the metal.

• Uniform attack is the most common form of corrosion. It is normally characterized by a chemical or electrochemical reaction which proceeds uniformly over the entire exposed surface or over a large area. The metal becomes thinner and eventually fails. For example, a piece of steel or zinc immersed in dilute sulfuric acid will normally dissolve at a uniform rate over its entire surface. A sheet iron roof will show essentially the same degree of rusting over its entire outside surface.

• Uniform attack, or general overall corrosion, represents the greatest destruction of metal on a tonnage basis. This form of corrosion, however, is not of too great concern from the technical standpoint, because the life of equipment can be accurately estimated on the basis of comparatively simple tests. Merely immersing specimens in the fluid involved is often sufficient.

Weathering steel or high strength low alloy (HSLA) steel has the unique characteristic that as it corrodes under proper conditions, it forms a dense and tightly adherent oxide barrier that seals out the atmosphere and retards further corrosion. This is in contrast to other steels that, as they corrode, they form a coarse, porous and flaky oxide that allows the atmosphere to continue penetrating the steel. Although cleaning and handling of the material can affect the short term appearance of the product, the overall corrosion resistance and ultimate appearance of weathering steel is not affected by cleanliness.

Corrosion Hues on a Weathering Steel

• The appearance of weathering steel or high strength low alloy (HSLA) steel may also be affected by other factors. During recrystallization the rust will trap particulate matter on the surface. If this material is colored it will contribute to the appearance of the rust. For example, in dirty industrial atmospheres the rust on weathering steel can be almost black due to the incorporation of airborne dirt. Chemical cleaning treatments such as acids can convert the hydrated iron oxide to other iron compounds of different color or appearance. In atmospheres with significant content of sulfur oxides deposits of white to yellow ferrous sulfate may appear in the rust on weathering steel.

• In some climates organic growth such as moss may be present and affect the appearance of the rust. Discolored areas on a weathering steel sculpture could be due to any of the variety of factors described above, or excessive corrosion. The rust layer on weathering steel in many climates does not consume a significant amount of steel in its formation, so removal in most cases should not affect the strength of the work. However, in some cases of inappropriate design crevices or pockets will trap water and the continual presence of water leads to excessive corrosion evidenced by rust flaking or observable metal loss.

Corrosion on Weathering Steel Lamppost

Uniform corrosion can be slowed or stopped by using the five basic facts:(1) Slow down or stop the movement of electrons

(a) Coat the surface with a non-conducting medium such as paint, lacquer or oil;

(b) Reduce the conductivity of the solution in contact with the metal an extreme case being to keep it dry. Wash away conductive pollutants regularly;

(c) Apply a current to the material (see cathodic protection).(2) Slow down or stop oxygen from reaching the surface. Difficult to do

completely but coatings can help.(3) Prevent the metal from giving up electrons by using a more corrosion

resistant metal higher in the electrochemical series. Use a sacrificial coating which gives up its electrons more easily than the metal being protected. Apply cathodic protection. Use inhibitors.

(4) Select a metal that forms an oxide that is protective and stops the reaction.(5) Control and consideration of environmental and thermal factors is also

essential.

Pitting corrosion is localized corrosion that occurs at microscopic defects on a metal surface. The pits are often found underneath surface deposits caused by corrosion product accumulation.

• Pitting is a form of extremely localized attack that results in holes in the metal. These holes may be small or large in diameter, but in most cases they are relatively small. Pits are sometimes isolated or so close together that they look like a rough surface. Generally a pit may be described as a cavity or hole with the surface diameter about the same as or less than the depth.

• Pitting is one of the most destructive and insidious forms of corrosion. It causes equipment to fail because of perforation with only a small percent weight loss of the entire structure. It is often difficult to detect pits because of their small size and because the pits are often covered with corrosion products. In addition, it is difficult to measure quantitatively and compare the extent of pitting because of the varying depths and numbers of pits that may occur under identical conditions. Pitting is also difficult to predict by laboratory tests. Sometimes the pits require a long time-several months or a year-to show up in actual service. Pitting is particularly vicious because it is a localized and intense form of corrosion, and failures often occur with extreme suddenness.

• Pitting corrosion can produce pits with their mouth open (uncovered) or covered with a semi-permeable membrane of corrosion products. Pits can be either hemispherical or cup-shaped.

Pitting is initiated by:• Localized chemical or mechanical damage to the protective oxide film;

water chemistry factors which can cause breakdown of a passive film are acidity, low dissolved oxygen concentrations (which tend to render a protective oxide film less stable) and high concentrations of chloride (as in seawater),

• Localized damage to, or poor application of, a protective coating ,• The presence of non-uniformities in the metal structure of the component,

e.g. nonmetallic inclusions.• Theoretically, a local cell that leads to the initiation of a pit can be caused

by an abnormal anodic site surrounded by normal surface which acts as a cathode, or by the presence of an abnormal cathodic site surrounded by a normal surface in which a pit will have disappeared due to corrosion.

• Pits can be either hemispherical or cup-shaped. In some cases they are flat-walled, revealing the crystal structure of the metal, or they may have a completely irregular shape. Pitting corrosion occurs when discrete areas of a material undergo rapid attack while most of the adjacent surface remains virtually unaffected. The following are common pit shapes divided in two groups:

• Trough Pits

Narrow, deep Shallow, wide

EllipticalVertical grain attack

• Sideway Pits

SubsurfaceUndercutting

Horizontal grain attack

Some definitions

• Pitting: Corrosion of a metal surface, confined to a point or small area, that takes the form of cavities.

• Pitting factor: Ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss.

• Pitting resistance equivalent number (PREN): An empirical relationship to predict the pitting resistance of austenitic and duplex stainless steels. It is expressed as PREN = Cr + 3.3 (Mo + 0.5 W) + 16N.

In the second case, post-examination should reveal the local cathode, since it will remain impervious to the corrosion attack as in the picture of an aluminum specimen shown below. Most cases of pitting are believed to be caused by local cathodic sites in an otherwise normal surface.

Control of pitting corrosion can be ensured by:

• Selecting a resistant material• Ensuring a high enough flow velocity of fluids in contact with the material

or frequent washing• Control of the chemistry of fluids and use of inhibitors• Use of a protective coating• Maintaining the material’s own protective film.

Note: Pits can be crack initiators in stressed components or those with residual stresses resulting from forming operations. This can lead to stress corrosion cracking.

Crevice or contact corrosion is the corrosion produced at the region of contact of metals with metals or metals with nonmetals. It may occur at washers, under barnacles, at sand grains, under applied protective films, and at pockets formed by threaded joints.

Full blown crevice in an otherwise very seawater resistant material

Crevice corrosion at rivets

Underside of panel where severe corrosion was found

Close-up picture showing the severity of corrosion

Crevice corrosion on aircraft

View of fuselage showing damage to protective coating

Crevice Corrosion induced by Zebra Mussels

Zebra mussels can be a real dragBiofouling is ubiquitous in the marine environment. There are two main categories of

fouling:

– noncalcareous (soft)– calcareous (hard)

• Algae, slime and hydroids exemplify noncalcareous foulers. Examples of calcareous foulers, which form shells comprised of calcium carbonate, are barnacles, encrusting bryozoans, mollusks, tubeworms and zebra mussels.

• Zebra mussels colonize pipes and constrict flow, therefore reducing the intake in heat exchangers, condensers, fire-fighting equipment, and air conditioning and cooling systems. Due to zebra mussel densities, the diameters of pipes have been reduced by two-thirds at water treatment facilities. Small mussels can get into engine cooling systems and cause overheating and damage. Navigational buoys have sunk under the weight of attached zebra mussels. Dock piling deterioration increases when encrusted with zebra mussels. Corrosion of steel and structural integrity of concrete can be affected by continued attachment of zebra mussels.

• The buildup of a heavy layer of zebra mussels on a surface can produce anaerobic conditions at the substrate. Microbiologically induced corrosion (MIC) is caused by many types of anaerobic bacteria. Stainless steel weldments are particularly susceptible to MIC. Oxygen concentration cells may be caused by zebra mussel infestations. This effect will establish corrosion cells, which may accelerate corrosion of both coated and bare ferrous substrates.

Pillowing of a lap joint

An advanced form of crevice corrosion is called pillowing. Notice how the rivet heads appear to be lower than the surrounding skin surface. During the investigation of the Aloha Boeing 737 aircraft incident, evidence was found of multiple site fatigue damage leading to structural failure.

The potential for crevice corrosion can be reduced by:

• Avoiding sharp corners and designing out stagnant areas;

• Use of sealants;

• Use welds instead of bolts or rivets;

• Selection of resistant materials.

Prevention of Crevice Corrosion

• The microstructure of metals and alloys is made up of grains, separated by grain boundaries. Intergranular corrosion is localized attack along the grain boundaries, or immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected. This form of corrosion is usually associated with chemical segregation effects (impurities have a tendency to be enriched at grain boundaries) or specific phases precipitated on the grain boundaries. Such precipitation can produce zones of reduced corrosion resistance in the immediate vicinity.

• The attack is usually related to the segregation of specific elements or the formation of a compound in the boundary. Corrosion then occurs by preferential attack on the grain-boundary phase, or in a zone adjacent to it that has lost an element necessary for adequate corrosion resistance - thus making the grain boundary zone anodic relative to the remainder of the surface. The attack usually progresses along a narrow path along the grain boundary and, in a severe case of grain-boundary corrosion, entire grains may be dislodged due to complete deterioration of their boundaries. In any case the mechanical properties of the structure will be seriously affected.

Many aluminum base alloys are susceptible to intergranular corrosion on account of either phases anodic to aluminum being present along grain boundaries or due to depleted zones of copper adjacent to grain boundaries in copper-containing alloys. Alloys that have been extruded or otherwise worked heavily, with a microstructure of elongated, flattened grains, are particularly prone to this damage.

The picture above shows a stainless steel which corroded in the heat affected zone a short distance from the weld. This is typical of intergranular corrosion in austenitic stainless steels. This corrosion can be eliminated by using stabilized stainless steels (321 or 347) or by using low-carbon stainless grades (304L or 3I6L).

Heat-treatable aluminum alloys (2000, 6000, and 7000 series alloys) can also have this problem. See the section on exfoliation corrosion below.

Exfoliation Corrosion

Exfoliation is a form of intergranular corrosion. It manifests itself by lifting up the surface grains of a metal by the force of expanding corrosion products occurring at the grain boundaries just below the surface. It is visible evidence of intergranular corrosion and most often seen on extruded sections where grain thickness is less than in rolled forms. This form of corrosion is common on aluminum, and it may occur on carbon steel.

The picture on the left shows exfoliation of aluminum. Exfoliation of carbon steel is apparent in the channel on the coating exposure panel on the right. The expansion of the metal caused by exfoliation corrosion can create stresses that bend or break connections and lead to structural failure.

A classic example is the sensitization of stainless steels or weld decay. Chromium-rich grain boundary precipitates lead to a local depletion of Cr immediately adjacent to these precipitates, leaving these areas vulnerable to corrosive attack in certain electrolytes. Reheating a welded component during multi-pass welding is a common cause of this problem. In austenitic stainless steels, titanium or niobium can react with carbon to form carbides in the heat affected zone (HAZ) causing a specific type of intergranular corrosion known as knife-line attack. These carbides build up next to the weld bead where they cannot diffuse due to rapid cooling of the weld metal. The problem of knife-line attack can be corrected by reheating the welded metal to allow diffusion to occur.

Intergranular corrosion of a failed aircraft component made of 7075-T6 aluminum (picture width = 500 mm)

It can be avoided by:

• Selection of stabilised materials

• Control of heat treatments and processing to avoid susceptible temperature range.

Prevention of Intergranular Corrosion

Dealloying is a rare form of corrosion found in copper alloys, gray cast iron, and some other alloys. Dealloying occurs when the alloy loses the active component of the metal and retains the more corrosion resistant component in a porous "sponge" on the metal surface. It can also occur by redeposition of the noble component of the alloy on the metal surface. Control is by the use of more resistant alloys-inhibited brasses and malleable or nodular cast iron.

The brass on the left dezincified leaving a porous copper plug on the surface. The gray cast iron water pipe shown on the right photo has graphitized and left graphitic surface plugs which can be seen on the cut surface. The rust tubercules or bubbles are also an indication of pitting corrosion.

The bottom photo shows a layer of copper on the surface of a dealloyed 70% copper-30% nickel cupronickel heat exchanger tube removed from a ship. Stagnant seawater is so corrosive that even this normally corrosion-resistant alloy has corroded. Virtually all copper alloys are subject to dealloying in some environments.

• There are several categories of hydrogen phenomena that are localized in nature. Atomic hydrogen, and not the molecule, is the smallest atom and it is small enough to diffuse readily through a metallic structure. When the crystal lattice is in contact or is saturated with atomic hydrogen, the mechanical properties of many metals and alloys are diminished. Nascent atomic hydrogen can be produced as a cathodic reaction when certain chemical species are present which act as negative catalysts (i.e. poisons) for the recombination of atomic to molecular hydrogen as shown in the following equation:

2 Ho H2(g)

• If the formation of molecular hydrogen is suppressed, the nascent atomic hydrogen can diffuse into the interstices of the metal instead of being harmlessly evolved as a gaseous reaction product. There are many chemical species which poison this recombination (e.g. cyanides, arsenic, antimony, or selenium compounds). However, the most commonly encountered species is hydrogen sulfide (H2S), which is formed in many natural decompositions, and in many petrochemical processes .

• Processes or conditions involving wet hydrogen sulfide, i.e. sour services, and the high incidence of sulfide-induced HIC has resulted in the term sulfide stress cracking (SSC). The SSC of medium strength steels has been a continuing source of trouble in the oil fields, and from these troubles has evolved in international standards. However, similar problems are encountered wherever wet hydrogen sulfide is encountered (e.g. acid gas scrubbing systems, heavy water plants, and waste water treatment).

• Failures have occurred in the field when storage tank roofs have become saturated with hydrogen by corrosion and then been subjected to a surge in pressure, resulting in the brittle failure of circumferential welds. In rare instances, even copper and Monel 400 (N04400) have been subjected to HIC. More resistant materials, such as Inconels and Hastelloys often employed to combat HIC, can become susceptible under the combined influence of severe cold work, the presence of hydrogen recombination poisons, and a direct current from the galvanic couple due to electrical contact with a more anodic member.

Continued

• The mechanism of HIC has not been definitely established. Various factors are believed to contribute to unlocking the lattice of the metal, such as hydrogen pressure at the crack tip, the competition of hydrogen atoms for the lattice bonding electrons, the easier plastic flow and dislocation formation in the metal at the crack tip in the presence of hydrogen, and the formation of certain metal hydrides in the alloy. The following phenomena have also been commonly reported in relation to hydrogen weakening of metallic components.

Continued

• The embrittlement of metal or alloy by atomic hydrogen involves the ingress of hydrogen into a component, an event that can seriously reduce the ductility and load-bearing capacity, cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Hydrogen embrittlement occurs in a number of forms but the common features are an applied tensile stress and hydrogen dissolved in the metal.

• Hydrogen blistering can occur when hydrogen enters steel as a result of the reduction reaction on a metal cathode. Single-atom nacent hydrogen atoms then diffuse through the metal until they meet with another atom, usually at inclusions or defects in the metal. The resultant diatomic hydrogen molecules are then too big to migrate and become trapped. Eventually a gas blister builds up and may split the metal as shown in the picture below:

Hydrogen blistering is controlled by minimizing corrosion in acidic environments. It is not a problem in neutral or caustic environments or with high-quality steels that have low impurity and inclusion levels.

The broken spring above on the left was brought to the KSC Materials Laboratory for failure analysis. Examination at high magnification in the scanning electron microscope (above right) revealed intergranular cleavage characteristic of hydrogen assisted cracking (hydrogen embrittlement). The part was zinc plated during refurbishment, and the hydrogen which entered the metal during the plating process had not been baked out. A postplating bakeout procedure should be standard for high strength steels.

Sources of Hydrogen • Sources of hydrogen causing embrittlement have been encountered in the making of

steel, in processing parts, in welding, in storage or containment of hydrogen gas, and related to hydrogen as a contaminant in the environment that is often a by-product of general corrosion. It is the latter that concerns the nuclear industry. Hydrogen may be produced by corrosion reactions such as rusting, cathodic protection, and electroplating. Hydrogen may also be added to reactor coolant to remove oxygen from reactor coolant systems. Hydrogen entry, the obvious pre-requisite of embrittlement, can be facilitated in a number of ways summarized below: (Defence Standard 03-30)

a) by some manufacturing operations such as welding, electroplating, phosphating and pickling; if a material subject to such operations is susceptible to hydrogen embrittlement then a final, baking heat treatment to expel any hydrogen is employed

b) as a by-product of a corrosion reaction such as in circumstances when the hydrogen production reaction described here acts as the cathodic reaction since some of the hydrogen produced may enter the metal in atomic form rather than be all evolved as a gas into the surrounding environment. In this situation, cracking failures can often be thought of as a type of stress corrosion cracking. If the presence of hydrogen sulfide causes entry of hydrogen into the component, the cracking phenomenon is often termed “sulphide stress cracking (SSC)”

c) the use of cathodic protection for corrosion protection if the process is not properly controlled.

Hydrogen Embrittlement of Mechanical Fasteners

• Hydrogen embrittlement of fasteners is a major factor in the choice of material or coating for such components. Hydrogen embrittlement may be a serious concern with high strength fasteners made of carbon and alloy steels for which is can be caused by the absorption of atomic hydrogen into the fastener's surface during manufacture and processing. The introduction of atomic hydrogen is particularly possible during acid pickling and alkaline cleaning prior to plating, and then during actual electroplating.

• The metallic coating subsequently plated on the fastener entraps atomic hydrogen in the base metal and if the hydrogen is not relieved by a post-baking operation the hydrogen atoms may migrate towards points of highest stress concentration when load or stress is applied. Cracks will promulgate through the component surface, weakening the component due to the loss of cross-section area. The failure is usually completed by a ductile fracture.

• The susceptibility of any material to hydrogen embrittlement in a given test is directly related to the characteristics of its trap population related to the material microstructure, dislocations, carbides and other elements present in the structure.

• The greater the hydrogen concentration becomes, the lower the critical stress, or lower the hydrogen concentration, the higher the critical stress at which failure may occur. Products having Vickers hardness exceeding HV 320 require special care to reduce the risk of this phenomenon during the plating process or coating procedures. Some experts feel that hardness exceeding HV 390 is a threshold beyond which further steps to manage hydrogen embrittlement risk are required.

• A stress relieving anneal should be considered for fasteners which have been work hardened during fabrication and are to be electroplated. Instances have been reported of fasteners failing by hydrogen embrittlement after many years in service with the cracks associated with corroded thread roots, providing thus an indication of the role of corrosion as a possible source of the hydrogen necessary to promote hydrogen embrittlement. (Defence Standard 03-30)

• Assuming that the fasteners are not charged with hydrogen before entering service, there are some features connected with atmospheric corrosion that indicate that this should not normally promote hydrogen embrittlement; since the condensed water films will often be insufficiently acidic to support a significant hydrogen-production cathodic reaction described here. On the other hand, in the occluded zones of crevices, acidic conditions are more likely and a periodic visual inspection programme is recommended with a policy of replacement of high-strength fasteners that are exhibiting significant corrosion.

Erosion corrosion is the result of a combination of an aggressive chemical environment and high fluid-surface velocities. This can be the result of fast fluid flow past a stationary object, such as the case with the oil-field check valve shown on the left below, or it can result from the quick motion of an object in a stationary fluid, such as happens when a ship's propeller churns the ocean.

Surfaces which have undergone erosion corrosion are generally fairly clean, unlike the surfaces from many other forms of corrosion.

• Erosion corrosion can be controlled by the use of harder alloys (including flame-sprayed or welded hard facings) or by using a more corrosion resistant alloy. Alterations in fluid velocity and changes in flow patterns can also reduce the effects of erosion corrosion.

• Erosion corrosion is often the result of the wearing away of a protective scale or coating on the metal surface. The oilfield production tubing shown above on the right corroded when the pressure on the well became low enough to cause multiphase fluid flow. The impact of collapsing gas bubbles caused the damage at joints where the tubing was connected and turbulence was greater.

• Many people assume that erosion corrosion is associated with turbulent flow. This is true, because all practical piping systems require turbulent flow-the fluid would not flow fast enough if lamellar (nonturbulent) flow were maintained. Most, if not all, erosion corrosion can be attributed to multiphase fluid flow. The check valve on the left above failed due to sand and other particles in an otherwise noncorrosive fluid. The tubing on the right failed due to the pressure differences caused when gas bubbles collapsed against the pipe wall and destroyed the protective mineral scale that was limiting corrosion.

• Cavitation occurs when a fluid's operational pressure drops below it's vapor pressure causing gas pockets and bubbles to form and collapse. This can occur in what can be a rather explosive and dramatic fashion. In fact, this can actually produce steam at the suction of a pump in a matter of minutes. When a process fluid is supposed to be water in the 20-35°C range, this is entirely unacceptable. Additionally, this condition can form an airlock, which prevents any incoming fluid from offering cooling effects, further exacerbating the problem. The locations where this is most likely to occur, such as:

o At the suction of a pump, especially if operating near the net positive suction head required (NPSHR)

o At the discharge of a valve or regulator, especially when operating in a near-closed position

o At other geometry-affected flow areas such as pipe elbows and expansions o Also, by processes incurring sudden expansion, which can lead to dramatic

pressure drops

• This form of corrosion will eat out the volutes and impellers of centrifugal pumps with ultrapure water as the fluid. It will eat valve seats. It will contribute to other forms of erosion corrosion, such as found in elbows and tees. Cavitation should be designed out by reducing hydrodynamic pressure gradients and designing to avoid pressure drops below the vapor pressure of the liquid and air ingress. The use of resilient coatings and cathodic protection can also be considered as supplementary control methods.

Cavitation corrosion of a deaerator

• Impingement attack is related to cavitation damage, and has been defined as ‘localized erosion-corrosion caused by turbulence or impinging flow.’ Entrained air bubbles tend to accelerate this action, as do suspended solids. This type of corrosion occurs in pumps, valves, orifices, on `heat-exchanger tubes, and at elbows and tees in pipelines.

• Impingement corrosion usually produces a pattern of localized attack with directional features. The pits or grooves tend to be undercut on the side away from the source of flow, in the same way that a sandy river bank at a bend in the river is undercut by the oncoming water.

• When a liquid is flowing over a surface (e.g. in a pipe), there is usually a critical velocity below which impingement does not occur and above which it increases rapidly. Impingement attack first received attention due to the poor behavior of some copper alloys in seawater.

• In practice, impingement and cavitation may occur together, and the resulting damage can be the result of both. Impingement may damage a protective oxide film and cause corrosion, or it may mechanically wear away the surface film to produce a deep groove.

• Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment. The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses, see an example of SCC of an aircraft component. The problem itself can be quite complex. The situation with buried pipelines is a good example of such complexity. The impact is most commonly catastrophic but rarely as it was for the historical failure of the UK Flixborough chemical reactor in 1974.

• Cold deformation and forming, welding, heat treatment, machining and grinding can introduce residual stresses. The magnitude and importance of such stresses is often underestimated. The residual stresses set up as a result of welding operations tend to approach the yield strength. The build-up of corrosion products in confined spaces can also generate significant stresses and should not be overlooked. SCC usually occurs in certain specific alloy-environment-stress combinations.

• Usually, most of the surface remains unattacked, but with fine cracks penetrating into the material. In the microstructure, these cracks can have an intergranular or a transgranular morphology. Macroscopically, SCC fractures have a brittle appearance. SCC is classified as a catastrophic form of corrosion, as the detection of such fine cracks can be very difficult and the damage not easily predicted. Experimental SCC data is notorious for a wide range of scatter. A disastrous failure may occur unexpectedly, with minimal overall material loss.

The micrograph below (X500) illustrates intergranular SCC of an Inconel heat exchanger tube with the crack following the grain boundaries.

The micrograph below(X300) illustrates SCC in a 316 stainless steel chemical processing piping system.Chloride stress corrosion cracking in austenitic stainless steel is characterized by the multi-branched "lightning bolt" transgranular crack pattern.

Stress corrosion crack in a bronze monument caused by build-up of rust around a decorative

steel rod.

• The most effective means of preventing SCC are:

1. properly with the right materials; 2. reduce stresses; 3. remove critical environmental species such as hydroxides, chlorides,

and oxygen; 4. and avoid stagnant areas and crevices in heat exchangers where

chloride and hydroxide might become concentrated.

Low alloy steels are less susceptible than high alloy steels, but they are subject to SCC in water containing chloride ions.

Chloride SCC One of the most important forms of stress corrosion that concerns the

nuclear industry is chloride stress corrosion. Chloride stress corrosion is a type of intergranular corrosion and occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions, and high temperature. It is thought to start with chromium carbide deposits along grain boundaries that leave the metal open to corrosion. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and use of low carbon steels.

Caustic SCC Despite the extensive qualification of Inconel for specific applications, a

number of corrosion problems have arisen with Inconel tubing. Improved resistance to caustic stress corrosion cracking can be given to Inconel by heat treating it at 620oC to 705oC, depending upon prior solution treating temperature. Other problems that have been observed with Inconel include wastage, tube denting, pitting, and intergranular attack.

What is SCC?SCC is the conjoint action of stress and a corrosive environment which leads to the

formation of a crack which would not have developed by the action of the stress or environment alone.

Why is it a problem?Because, it can happen ‘unexpectedly’ and rapidly after a period of satisfactory serviceleading to catastrophic failure of structures or leaks in pipework.

Where does it occur typically?Typical SCC failures are seen in pressure vessels, pipework, highly stressed

components and in systems when an excursion from normal operating conditions or the environment occurs.

Where do the stresses come from?The stresses that cause SCC are either produced as a result of the use of the component

in service or residual stresses introduced during manufacturing.

Where does the corrosive environment come from?The environment is either the permanent service environment i.e. sea water or a

temporary one caused by operations such as cleaning of the system which can leave a residue, or if the stress is applied during the operation initiate cracking.

How is this different from ‘normal’ corrosion?SCC is a corrosion mechanism that requires the pairing of a material with a very

particular environment and the application of a tensile stress above a critical value. Corrosion can occur in other environments without SCC. Examples of well-known material/environment pairs are:

MATERIAL ENVIRONMENT Brass Ammonia Stainless steel Chlorides High strength steels Hydrogen

How can SCC be controlled?

• By selecting a material that is not susceptible to the service environment and by ensuring that any changes to the environment caused by cleaning etc are not detrimental.

• By controlling the service stresses through careful design and minimizing stress concentrations to keep them below the critical value. Residual stresses can be reduced by heat treatments and careful design for manufacturing.

• By using corrosion inhibitors during cleaning operations or to control the environment in a closed system.

• By coating the material and effectively isolating the material from the environment.

• Galvanic corrosion is an electrochemical action of two dissimilar metals in the presence of an electrolyte and an electron conductive path. It occurs when dissimilar metals are in contact.

• It is recognizable by the presence of a buildup of corrosion at the joint between the dissimilar metals. For example, when aluminum alloys or magnesium alloys are in contact with steel (carbon steel or stainless steel), galvanic corrosion can occur and accelerate the corrosion of the aluminum or magnesium. This can be seen on the photo above where the aluminum helicopter blade has corroded near where it was in contact with a steel counterbalance.

Galvanic Series In Sea Water Platinum

GoldGraphiteSilver18-8-3 Stainless steel, type 316 (passive)18-8 Stainless steel, type 304 (passive)Titanium13 percent chromium stainless steel, type 410 (passive)7NI-33Cu alloy75NI-16Cr-7Fe alloy (passive)Nickel (passive)Silver solderM-BronzeG-Bronze70-30 cupro-nickelSilicon bronzeCopperRed brassAluminum bronzeAdmiralty brassYellow brass76NI-16Cr-7Fe alloy (active)Nickel (active)Naval brassManganese bronzeMuntz metalTinLead18-8-3 Stainless steel, type 316 (active)18-8 Stainless steel, type 304 (active)13 percent chromium stainless steel, type 410 (active)Cast ironMild steelAluminum 2024CadmiumAlcladAluminum 6053Galvanized steelZincMagnesium alloysMagnesium

Noble (least Active)

Anodic (Most Active)

• The natural differences in metal potentials produce galvanic differences, such as the galvanic series in sea water. If electrical contact is made between any two of these materials in the presence of an electrolyte, current must flow between them. The farther apart the metals are in the galvanic series, the greater the galvanic corrosion effect or rate will be. Metals or alloys at the upper end are noble while those at the lower end are active. The more active metal is the anode or the one that will corrode.

• Control of galvanic corrosion is achieved by using metals closer to each other in the galvanic series or by electrically isolating metals from each other. Cathodic protection can also be used to control galvanic corrosion effects.

• The scuba tank above suffered galvanic corrosion when the brass valve and the steel tank were wetted by condensation. Electrical isolation flanges like those shown on the right are used to prevent galvanic corrosion. Insulating gaskets, usually polymers, are inserted between the flanges, and insulating sleeves and washers isolate the bolted connections.

• KSC conducts research on the effects of galvanic corrosion. The photo below shows the corrosion caused by a stainless steel screw causing galvanic corrosion of aluminum. The picture shows the corrosion resulting from only six months exposure at the Atmospheric Test Site.

• Corrosion fatigue is a special case of stress corrosion caused by the combined effects of cyclic stress and corrosion. No metal is immune from some reduction of its resistance to cyclic stressing if the metal is in a corrosive environment. Damage from corrosion fatigue is greater than the sum of the damage from both cyclic stresses and corrosion.  Control of corrosion fatigue can be accomplished by either lowering the cyclic stresses or by corrosion control.

• The "beach marks" on the propeller shown below mark the progression of fatigue on this surface.

An infamous example of corrosion fatigue occured in 1988 on an airliner flying between the Hawaiian islands. This disaster, which cost one life, prompted the airlines to look at their airplanes and inspect for corrosion fatigue.

• Similar beach marks are shown on the aerospace part below left. The high magnification scanning electron microscope image on the right shows striations (individual crack progression marks). The part shown below is also discussed in the section on fretting corrosion.

• The picture on the left shows cracking and staining of a seawall . The pitting corrosion in the right photo occurred on an aluminum railing on a concrete causeway .

• Concrete is a widely-used structural material that is frequently reinforced with carbon steel reinforcing rods, post-tensioning cable or prestressing wires. The steel is necessary to maintain the strength of the structure, but it is subject to corrosion. The cracking associated with corrosion in concrete is a major concern in areas with marine environments (like KSC) and in areas which use deicing salts.

• There are two theories on how corrosion in concrete occurs:

1. Salts and other chemicals enter the concrete and cause corrosion. Corrosion of the metal leads to expansive forces that cause cracking of the concrete structure.

2. Cracks in the concrete allow moisture and salts to reach the metal surface and cause corrosion.

• Both possibilities have their advocates, and it is also possible that corrosion in concrete can occur either way. The mechanism isn't truly important, the corrosion leads to damage, and the damage must be controlled.

• In new construction, corrosion in concrete is usually controlled by embedding the steel deep enough so that chemicals from the surface don't reach the steel (adequate depth of cover). Other controls include keeping the water/cement ratio below 0.4, having a high cement factor, proper detailing to prevent cracking and ponding, and the use of chemical admixtures. These methods are very effective, and most concrete structures, even in marine environments, do not corrode.

• Unfortunately, some concrete structures do corrode. When this happens, remedial action can include repairing the cracked and spalled concrete, coating the surface to prevent further entry of corrosive chemicals into the structure, and cathodic protection, an electrical means of corrosion control. KSC has experience with all of these methods of controlling corrosion on existing concrete structures.

Concentration cell corrosion occurs when two or more areas of a metal surface are in contact with different concentrations of the same solution. There are three general types of concentration cell corrosion:

1. metal ion concentration cells 2. oxygen concentration cells, and 3. active-passive cells.

Metal Ion Concentration Cells In the presence of water, a high concentration of metal ions will exist under

faying surfaces and a low concentration of metal ions will exist adjacent to the crevice created by the faying surfaces. An electrical potential will exist between the two points. The area of the metal in contact with the low concentration of metal ions will be cathodic and will be protected, and the area of metal in contact with the high metal ion concentration will be anodic and corroded. This condition can be eliminated by sealing the faying surfaces in a manner to exclude moisture. Proper protective coating application with inorganic zinc primers is also effective in reducing faying surface corrosion.

Oxygen Concentration Cells A water solution in contact with the metal surface will normally contain dissolved

oxygen. An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse uniformly into the solution, thereby creating a difference in oxygen concentration between two points. Typical locations of oxygen concentration cells are under either metallic or nonmetallic deposits (dirt) on the metal surface and under faying surfaces such as riveted lap joints. Oxygen cells can also develop under gaskets, wood, rubber, plastic tape, and other materials in contact with the metal surface. Corrosion will occur at the area of low-oxygen concentration (anode). The severity of corrosion due to these conditions can be minimized by sealing, maintaining surfaces clean, and avoiding the use of material that permits wicking of moisture between faying surfaces.

Active-Passive Cells Metals that depend on a tightly adhering passive film (usually an oxide) for corrosion

protection; e.g., austenitic corrosion-resistant steel, can be corroded by active-passive cells. The corrosive action usually starts as an oxygen concentration cell; e.g., salt deposits on the metal surface in the presence of water containing oxygen can create the oxygen cell. If the passive film is broken beneath the salt deposit, the active metal beneath the film will be exposed to corrosive attack. An electrical potential will develop between the large area of the cathode (passive film) and the small area of the anode (active metal). Rapid pitting of the active metal will result. This type of corrosion can be avoided by frequent cleaning and by application of protective coatings

This type of corrosion occurs under painted or plated surfaces when moisture permeates the coating. Lacquers and "quick-dry" paints are most susceptible to the problem. Their use should be avoided unless absence of an adverse effect has been proven by field experience. Where a coating is required, it should exhibit low water vapor transmission characteristics and excellent adhesion. Zinc-rich coatings should also be considered for coating carbon steel because of their cathodic protection quality.

Filiform corrosion normally starts at small, sometimes microscopic, defects in the coating. Lacquers and "quick-dry" paints are most susceptible to the problem. Their use should be avoided unless absence of an adverse effect has been proven by field experience. Where a coating is required, it should exhibit low water vapor transmission characteristics and excellent adhesion. Zinc-rich coatings should also be considered for coating carbon steel because of their cathodic protection quality.

The picture on the left shows filiform corrosion causing bleed-through on a welded tank. The picture on the right shows "worm-like" filiform corrosion tunnels forming under a coating at the Atmospheric Test Site.

Filiform corrosion is minimized by careful surface preparation prior to coating, by the use of coatings that are resistant to this form of corrosion (see above), and by careful inspection of coatings to insure that holidays, or holes, in the coating are minimized.

• Microbial corrosion (also called microbiologically-influenced corrosion or MIC) is corrosion that is caused by the presence and activities of microbes. This corrosion can take many forms and can be controlled by biocides or by conventional corrosion control methods.

• There are a number of mechanisms associated with this form of corrosion, and detailed explanations are available at the web sites listed at the bottom of this section. Most MIC takes the form of pits that form underneath colonies of living organic matter and mineral and biodeposits. This biofilm creates a protective environment where conditions can become quite corrosive and corrosion is accelerated.

• The picture below shows a biofilm on a metallic condenser surface. These biofilms can allow

• corrosive chemicals to collect within and under the films. Thus the corrosive conditions under a biofilm can be very aggressive, even in locations where the bulk environment is noncorrosive.

MIC can be a serious problem in stagnant water systems such as the fire-protection system that produced the pits shown above. (see Pitting Corrosion). The use of biocides and mechanical cleaning methods can reduce MIC, but anywhere where stagnant water is likely to collect is a location where MIC can occur.

• Corrosion (oxidation of metal) can only occur if some other chemical is present to be reduced. In most environments, the chemical that is reduced is either dissolved oxygen or hydrogen ions in acids. In anaerobic conditions (no oxygen or air present), some bacteria (anaerobic bacteria) can thrive. These bacteria can provide the reducible chemicals that allow corrosion to occur. That's how the limited corrosion that was found on the hull of the Titanic occurred. The picture below shows a "rusticle" removed from the hull of Titanic. This combination of rust and organic debris clearly shows the location of rivet holes and where two steel plates overlapped.

• Much microbial corrosion involves anaerobic or stagnant conditions, but it can also be found on structures exposed to air. The pictures below show a spillway gate from a hydroelectric dam on the Columbia River.

The stress corrosion cracks were caused by pigeon droppings which produced ammonia-a chemical that causes stress corrosion cracking on copper alloys like the washers used on this structure. Since it's impossible to potty train pigeons, a new alloy resistant to ammonia was necessary.

In addition to the use of corrosion resistant alloys, control of MIC involves the use of biocides and cleaning methods that remove deposits from metal surfaces. Bacteria are very small, and it is often very difficult to get a metal system smooth enough and clean enough to prevent MIC.