CORROSION FAILURES

THIRUNAVUKARASU.H13MY12

Content

• Over view of various types of corrosion .• Factor influencing corrosion failures .• Analysis of corrosion failure

Definition of Corrosion

• Corrosion is the deterioration of materials by chemical interaction with their environment. 

• unstable due to higher energy comes to lower state.

Classification of corrosion 1. wet corrosion 2. Dry corrosion

8 Forms of Corrosion:

• Uniform• Pitting• Crevice Corrosion or Concentration Cell• Galvanic or Two-Metal• Stress Corrosion Cracking• Intergranular• Dealloying• Erosion Corrosion

Uniform corrosion

• Uniform corrosion is characterized by corrosive attack proceeding evenly over the entire surface area, or a large fraction of the total area.

Pitting corrosion• Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the

creation of small holes in the metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic while an unknown but potentially vast area becomes cathodic, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with limited diffusion of ions. The mechanism of pitting corrosion is

probably the same as crevice corrosion.

• 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:

• metal ion concentration cells• oxygen concentration cells, and• active-passive cells.

Crevice Corrosion or Concentration Cell

Galvanic Corrosion:• Possibility when two dissimilar metals are electrically

connected in an electrolyte• Results from a difference in oxidation potentials of metallic

ions between two or more metals. The greater the difference in oxidation potential, the greater the galvanic corrosion.

• The less noble metal will corrode (i.e. will act as the anode) and the more noble metal will not corrode (acts as cathode).

• Perhaps the best known of all corrosion types is galvanic corrosion, which occurs at the contact point of two metals or alloys with different electrode potentials.

GALVANIC SERIESGalvanic Series in Seawater (supplements Faraq Table 3.1 , page 65), EIT Review Manual, page 38-2Tendency to be protected from corrosion, cathodic, more noble end

MercuryPlatinumGoldZirconium GraphiteTitaniumHastelloy C MonelStainless Steel (316-passive)Stainless Steel (304-passive)Stainless Steel (400-passive)Nickel (passive oxide)SilverHastelloy 62Ni, 17CrSilver solder Inconel 61Ni, 17CrAluminum (passive AI203)70/30 copper-nickel90/10 copper-nickelBronze (copper/tin)CopperBrass (copper/zinc)Alum Bronze Admiralty BrassNickelNaval Brass TinLead-tinLeadHastelloy AStainless Steel (active)316 404 430 410Lead Tin SolderCast ironLow-carbon steel (mild steel)Manganese UraniumAluminum AlloysCadmiumAluminum ZincBerylliumMagnesium

PASSIVE – will not corrode – act as cathode. These elements are least likely to give up electrons!

ACTIVE – will corrode – act as anode. These elements most likely to give up electrons!

Big Cathode, Small Anode = Big Trouble

Design for Galvanic Corrosion?

• Material Selection: Do not connect dissimilar metals! Or if you can’t avoid it:– Try to electrically isolate one from the other

(rubber gasket).– Make the anode large and the cathode small• Bad situation: Steel siding with aluminum fasteners• Better: Aluminum siding with steel fasteners

• Eliminate electrolyte• Galvanic of anodic protection

Design for Galvanic Corrosion?

• Galvanic severity depends on:– NOT• Not amount of contact• Not volume• Not mass

– Amount of separation in the galvanic series– Relative surface areas of the two. Severe

corrosion if anode area (area eaten away) is smaller than the cathode area. Example: dry cell battery

Steel bolt (less noble) is isolated from copper plates.

Stress Corrosion Cracking:

• Spontaneous corrosion induced cracking of a material under static (or residual) tensile stress.

• Problem with parts that have residual stress – stamping double whammy – residual stress at bends = SCC + stress concentration.

• Also known as environmentally assisted cracking (EAC), other forms:– Hydrogen embrittlement– Caustic embrittlement– Liquid metal corrosion

Design for Stress Corrosion Cracking:

• Reduce applied or residual stress - Stress relieve to eliminate residual stress (i.e. stress relieve after heat treat).

• Introduce residual compressive stress in the service.

• Use corrosion alloy inhibitors.• Apply protective coatings.

Stress Corrosion Cracking:

Intergranular Attack:

• Corrosion which occurs preferentially at grain boundries.

• Why at grain boundries?– Higher energy areas which may be more anodic

than the grains.– The alloy chemistry might make the grain

boundries dissimilar to the grains. The grain can act as the cathode and material surrounding it the anode.

Intergranular Attack:

• How to recognize it?– Near surface– Corrosion only at grain boundries (note if only a

few gb are attacked probably pitting)– Corrosion normally at uniform depth for all grains.

Example 1: Intergranular Attack:

• Sensitization of stainless steels:– Heating up of austenitic stainless steel (750 to

1600 F) causes chromuim carbide to form in the grains. Chromuim is therefore depleted near the grain boundries causing the material in this area to essentially act like a low-alloy steel which is anodic to the chromium rich grains.

Example: Intergranular Attack:

• Sensitization of stainless steels:– Heating up of austenitic stainless steel (750 to

1600 F) causes chromuim carbide to form in the grains. Chromuim is therefore depleted near the grain boundries causing the material in this area to essentially act like a low-alloy steel which is anodic to the chromium rich grains.

– Preferential Intergranular Corrosion will occur parallel to the grain boundary – eventually grain boundary will simply fall out!!

Design for Intergranular Attack:

• Watch welding of stainless steels (causes sensitization). Always anneal at 1900 – 2000 F after welding to redistribute Cr.

• Use low carbon grade stainless to eliminate sensitization (304L or 316L).

• Add alloy stabilizers like titanium which ties up the carbon atoms and prevents chromium depletion.

Intergranular Attack:

Dealloying:• When one element in an alloy is anodic to the other

element.• Example: Removal of zinc from brass (called

dezincification) leaves spongy, weak brass.• Brass alloy of zinc and copper and zinc is anodic to

copper (see galvanic series).

Dealloying:• Danger!– The alloy may not appear damaged– May be no dimensional variations– Material generally becomes weak – hidden to inspection!

Dealloying:• Two common types:– Dezincification – preferential removal of zinc in brass

• Try to limit Zinc to 15% or less and add 1% tin.• Cathodic protection

– Graphitization – preferential removal of Fe in Cast Iron leaving graphite (C).

EROSION-CORROSION (“Flow-Assisted” or “Flow-Accelerated” Corrosion)

• An increase in rate of attack on a metal because

of relative movement between a corrosive fluid and the metal

surface.

Removal of the metal may be:

– as corrosion product which “removes” the surface because of the high

fluid shear and bares the metal beneath;

– as metal ions, which are swept away by the fluid flow before they can

deposit as corrosion product.

EROSION-CORROSION

Remember the distinction between erosion-corrosion and

erosion:

– erosion is the straightforward wearing away by the

mechanical abrasion caused by suspended particles . . . e.g.,

sand-blasting, erosion of turbine blades by droplets . . .

– erosion-corrosion also involves a corrosive environment . . .

the metal undergoes a chemical reaction.

EROSION-CORROSIONErosion-corrosion produces a distinctive surface finish:grooves, waves, gullies, holes, etc., all oriented with respect to the fluid flow pattern . . .

Erosion-corrosion of condenser tube wall.

EROSION-CORROSIONVulnerable equipment includes:

• - pipes (bends, elbows, tees);• - valves;• - pumps;• - blowers;• - propellers, impellers;• - stirrers;• - stirred vessels;• - flow-measuring orifices, venturies;• - turbine blades;• - nozzles;• - metal-working equipment (scrapers, cutters, • grinders, mills);• - spray impingement components;• - etc.

EROSION-CORROSION

Prevention of Erosion-Corrosion

• design (avoid impingement geometries, high velocity, etc.);

• Corrosion free environment.

• Materials selection (use Cr-containing steels);

• use corrosion-resistant coatings.

Methods to Control Corrosion

There are five methods to control corrosion: material selection coatings changing the environment changing the potential design

How to avoid (or control) Corrosion?

• Material Selection! Remember – environment key. Look at potential pH diagrams!!

• Galvanic - Avoid using dissimilar metals.– Or close together as possible– Or MAKE ANODE BIG!!!

How to avoid (or control) Corrosion?

• Pitting/Crevice: Watch for stagnate water/ electrolyte.– Use gaskets– Use good welding practices

• Intergranular – watch grain size, environment, temperature, etc.. Careful with Stainless Steels and AL.

How to avoid (or control) Corrosion?

• Consider organic coating (paint, ceramic, chrome, etc.) – DANGER IF IT GETS SCRACTHED!!

• OR BETTER YET, consider cathodic protection:– such as zinc (or galvanized) plating on steel– Mg sacrificial anode on steel boat hull

DESIGN for Corrosion

Surface Treatment (Coatings)

• Organic paints• Chromating and phosphating:– The Process - chromating and phosphating are

surface-coating processes that enhance the corrosion resistance of metals. Both involve soaking the component in a heated bath based on chromic or phosphoric acids. The acid reacts with the surface, dissolving some of the surface metal and depositing a thin protective layer of complex chromium or phosphorous compounds

Material Selection:

• Noble material are selected . Which reduces the corrosion

Cathodic Protection (CP)• Cathodic protection (CP) is a technique to control

the corrosion of a metal surface by making it work as a cathode of an electrochemical cell. This is achieved by placing in contact with the metal to be protected another more easily corroded metal to act as the anode of the electrochemical cell. Cathodic protection systems are most commonly used to protect steel, water or fuel pipelines and storage tanks, steel pier piles, ships, offshore oil platforms and onshore oil well casings.

Factors influencing corrosion

• Nature of metal.

• Nature of corrosive environment.

1.Nature of Metal:Purity of metal:

• Impure the metal, more is the rate of corrosion. • This is because impurities act as “minute

electrochemical cells.”

Physical state of metal:

• Physical state of metal like grain size, stress affects the corrosion rate.

• Stressed areas undergo more corrosion.

Position in Galvanic series:

• More the oxidation potential i.e if metal is higher in the galvanic series.

• it is more anodic and hence more is the rate of corrosion.

Nature of Oxide layer:

• Corrosion rate depends upon the nature of oxide layer whether it is stable, unstable, porous etc.

Passive nature of metal:• Some metals like Ti, Al, Cr show more corrosion resistance than expected from their position in the electrochemical series.• These are called passive metals. • This is due to formation of thin protective layer on metal surface.

Nature of corrosive products:

If the corrosion products are soluble or volatile in nature, then corrosion increases.

Relative area of anode and cathode:

smaller the anodic area, more is the corrosion rate because oxidation of anode occurs at faster rate.

2. Nature of corroding environment:

Temperature: Rate of corrosion increases with increase in

temperature. Moisture:

More the moisture, more is the rate of corrosion.pH Value:

Lesser the pH i.e. acidic conditions, more is corrosion.

Nature of electrolyte: Presence of salts in the electrolyte increases the rate

of corrosion.

Presence of impurities in atmosphere:

• Impurities increase the rate of corrosion. • Corrosion is more in industrial areas and sea. • This is because of presence of gases like CO2, H2S, SO2

REFERENCES:

• Herbert H Uhlig and Winston Revie , "Corrosion and Corrosion Control – An Introduction to Corrosion Science and Engineering", Third Edition, John Wiley & Sons, 1985.

• Mars G.Fontana, "Corrosion Engineering", Third Edition, McGraw Hill Inc., 1987.

• Rajnarayan, "Metallic Corrosion and Prevention", Oxford Publications, 1988.

• Kenneth R Trethewey and John Chamberlain, "Corrosion for Science and Engineering", Second Edition, Longman Inc., 1996.

• Denny A. Jones, "Principles and Prevention of Corrosion", second edition, Prentice Hall Inc., 1996.

• ASM Hand book – Vol 13: Corrosion, ASM International, 2001.