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Erosion Corrosion

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

An increase in corrosion brought about by a high relative velocity between the corrosive environment and the surface.

Removal of the metal may be:– as corrosion product which “spalls off” 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.

N.B. 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.

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

Erosion-corrosion of condenser tube wall.

Erosion-corrosion of stainless alloy pump impeller.

Impeller lasted ~ 2 years in oxidizing conditions;after switch to reducing conditions, it lasted ~ 3 weeks!

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Most metals/alloys are susceptible to erosion-corrosion.Metals that rely on protective surface film for corrosion protection are particularly vulnerable, e.g.: Al

Pb SS CS.

Attack occurs when film cannot form because of erosion caused by suspended particles (for example), or when rate of film formation is less than rate of dissolution and transfer to bulk fluid.

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Erosion-Corrosion found in: - aqueous solutions;

- gases;- organic liquids;- liquid metal.

If fluid contains suspended solids, erosion-corrosion may be aggravated. Vulnerable equipment is that subjected to high-velocity fluid, to rapid

change in direction of fluid, to excessive turbulence . . .viz. equipment in which the contacting fluid has a very thin boundary

layer - high mass transfer rates.

Vulnerable equipment includes:- pipes (bends, elbows, tees);- valves;- pumps;- blowers;- propellers, impellers;- stirrers;- stirred vessels;- HX tubing (heaters, condensers);

- flow-measuring orifices, venturies;- turbine blades;- nozzles;- baffles;- metal-working equipment (scrapers, cutters, grinders, mills);- spray impingement components;- etc.

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Surface film effectsProtective corrosion-product films important for resistance to erosion-corrosion.Hard, dense, adherent, continuous films give good resistance, provided that they are not brittle and easily removed under stress.

Lead sulphate film protects lead against DILUTE H2SO4 under stagnant conditions, but not under rapidly moving conditions.

Erosion-corrosion of hard lead by 10% sulphuric acid (velocity 39 ft/sec).

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pH affects films in erosion-corrosion of low-alloy steel.

Scale generally granular Fe3O4 (non-protective). But at pH 6 & pH 10, scale Fe(OH)2/Fe(OH)3 . . . hinders mass transport of oxygen and ionic species.

Effect of pH of distilled water on erosion-corrosion of carbon steel at 50C (velocity 39 ft/sec).

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N.B. Dissolved O2 often increases erosion-corrosion . . .

e.g. copper alloys in seawater. . . BUT . . . on steels, dissolved O2 will inhibit erosion-corrosion . . . utilized in boiler feedwater systems.

Effects of temperature and dissolved O2 on the weight-loss of AISI 304 stainless steel exposed for 800 hours in flowing water at 3.7 m/s.

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Effect of oxygen dosing on erosion-corrosion and potential of carbon steel in water at 150C, pH at 25oC= 7.8.

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Good resistance of Ti to erosion-corrosion in:

- seawater;

- Cl- solutions;

- HNO3;

and many other environments.

Resistance depends on formation and stability of TiO2 films.

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Chromium imparts resistance to erosion-corrosion to: - steels; - Cu alloys.

Such tests have led to the marketing of a new alloy for condenser tubes . . “CA-722” . . . previously “IN-838” . . . with constituents . . . Cu-16Ni-0.4Cr.

Effect of chromium additions on seawater impingement-corrosion resistance of copper-nickel alloys. 36-day test with 7.5 m/s jet velocity; seawater temperature: 27C.

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Velocity Effects

N.B. Turbulent flow regime for V < Vc is sometimes called

Flow-Assisted Corrosion regime.

Schematic showing the effect of flow velocity on erosion-corrosion rate.

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Relationship between flow velocity, v, and erosion-corrosion rate, w, may be written as . . .

w = kva where k and a are constants that depend on the system.

DISCUSS: What happens when v = 0 ?How do we express no dependence on velocity?

The exponent a varies between . . .0.3 (laminar flow) and 0.5 (turbulent flow)...occasionally reaching > 1.0 for mass transfer and fluid shear

effects.

For mechanical removal of oxide films (spalling), the fluid shear stress at the surface is important, and a > 1.0 . . . (may reach 2 - 4).

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Erosion-Corrosion in Carbon Steel and Low-Alloy Steels

N.B. these materials are used extensively in boilers, turbines, feed-water heaters in fossil & nuclear plants, CANDU feeders.

High velocities occur in single-phase flow (water) and two-phase flow (wet steam).

Single-phase E-C seen in H.P. feedwater heaters, SG inlets in AGRs, feedwater pumps, and CANDU feeders.

Two-phase E-C more widespread . . . steam extraction piping, cross-over piping (HP turbine to moisture separator), steam side of feedwater heaters, and CANDU feeders.

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Material effects – low-alloy steel . . .

Erosion-corrosion loss as a function of time for mild steel and 1 Cr 0.5 Mo steel in water (pH at 25C = 9.05) flowing through an orifice at 130C.

Cr additions reduce E-C.

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Flow dependence (single phase)...

Erosion-corrosion rate of carbon steel as a function of flow rate of deoxygenated water through orifice at pH 9.05 and at 149C.

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Mechanism... for E-C of C.S. in high temperature de-oxygenated water...

- magnetite film dissolves reductively

Fe3O4 + (3n-4) H2O + 2e

3Fe(OH)n(2-n) + (3n-8)H+

- high mass transfer rates remove soluble Fe II species;

- oxide particles eroded from weakened film by fluid shear stress;

- metal dissolves to try and maintain film.

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Mass transfer characteristics correlated by expressions such as...

Sh = kRea Scb Sh = Sherwood Number =

Re = Re = Reynolds Number

Sc = Sc = Schmidt Number

Shear stress correlated by ….

= f f = friction factor

and at high Re, f independent of velocity so

D

kd

dv

D

2v

2v

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Temperature and pH dependence for single-phase E-C of CS . . .

Effect of temperature on the exponent of the mass transfer coefficient for the erosion-corrosion of carbon steel in flowing water at various pHs.

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Prevention of Erosion-Corrosion

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

• chemistry (e.g., in steam supply systems . . . for CS or low-alloy steel add O2, maintain pH > 9.2, use morpholine rather than NH3);

• materials (use Cr-containing steels);

• use hard, corrosion-resistant coatings.

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CAVITATION DAMAGE

Similar effect to E-C: mechanical removal of oxide film caused by collapsing vapour bubbles.

High-speed pressure oscillations (pumps, etc.) can create shock waves > 60,000 psi. Surface attack often resembles closely-spaced pitting.

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FRETTING CORROSIONSimilar to E-C but surface mechanical action provided by wear of another surface . . . generally intermittent, low-amplitude rubbing.Two theories . . . with same overall result . . .

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Effects in terms of materials COMBINATIONSFretting resistance of various materials

Source: J.R. McDowell, ASTM Special Tech. Pub. No. 144, p. 24, Philadelphia, 1952.

Poor Average Good

Aluminum on cast ironAluminum on stainless steelMagnesium on cast ironCast iron on chrome plateLaminated plastic on cast ironBakelite on cast ironHard tool steel on stainlessChrome plate on chrome plateCast iron on tin plateCast iron on cast iron with coating of shellac

Cast iron on cast ironCopper on cast ironBrass on cast ironZinc on cast ironCast iron on silver plateCast iron on silver plateCast iron on amalgamated copper plateCast iron on cast iron with rough surfaceMagnesium on copper plateZirconium on zirconium

Laminated plastic on gold plateHard tool steel on tool steelCold-rolled steel on cold- rolled steelCast iron on cast iron with phosphate coatingCast iron on cast iron with coating of rubber cementCast iron on cast iron with coating of tungsten sulfideCast iron on cast iron with rubber

gasketCast iron on cast iron with Molykote lubricantCast iron on stainless with Molykote lubricant

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Prevention of Fretting Corrosion

• lubricate;

• avoid relative motion (add packing, etc.);

• increase relative motion to reduce attack severity;

• select materials (e.g., choose harder component).

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Stress Corrosion

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STRESS CORROSION (“Stress Corrosion Cracking” - SCC)

Under tensile stress, and in a suitable environment, some metals and

alloys crack . . . usually, SCC noted by absence of significant surface

attack . . . occurs in “ductile” materials.

Stress Corrosion

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“Transgranular” SCC (“TGSCC”)

Cross section of stress-corrosioncrack in stainless steel.

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“Intergranular” SCC (“IGSCC”)

Intergranular stress corrosion cracking of brass.

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Two original classic examples of SCC:– “season cracking” of brass;– “caustic embrittlement” of CS;

both terms obsolete.

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“Season Cracking”Occurs where brass case is crimped onto bullet, i.e., in area of high residual stress.Common in warm, wet environments (e.g., tropics).Ammonia (from decomposition of organic matter, etc.) must be present.

Season cracking of German ammunition.

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“Caustic Embrittlement”Early steam boilers (19th and early 20th century) of riveted carbon steel. Both stationary and locomotive engines often exploded.

Examination showed:• cracks or brittle failures around rivet holes;• areas susceptible were cold worked by riveting (i.e., had high residual stresses);• whitish deposits in cracked regions were mostly caustic (i.e., sodium hydroxide

from chemical treatment of boiler water); • small leaks at rivets would concentrate NaOH and even dry out to solid. SCC

revealed by dye penetrant.

Carbon steel plate from a caustic storage tank failed by caustic embrittlement.

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Factors important in SCC:

• environmental composition;• stress;• metal composition and microstructure;• temperature;

e.g., brasses crack in NH3, not in Cl-;

SSs crack in Cl-, not in NH3;

SSs crack in caustic, not in H2SO4, HNO3, CH3COOH, . . . etc.

}necessary

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STRESS

The greater the stress on the material, the quicker it will crack.

(N.B. in fabricated components, there are usually RESIDUAL STRESSES

from cold working, welding, surface treatment such as grinding or shot

peening, etc., as well as APPLIED STRESSES from the service, such as

hydrostatic, vapour pressure of contents, bending loads, etc.).

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Composite curves illustrating the relative stress-corrosion-cracking resistancefor commercial stainless steels in boiling 42% magnesium chloride.

DISCUSS:how would you obtain such a curve and what does it mean?

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The MAXIMUM stress you can apply before SCC is formed (c.f.

MINIMUM stress to be applied compressively to prevent SCC) depends on

alloy (composition and structure), temperature, and environment

composition.

Such “THRESHOLD” stresses may be between 10% & 70% of the yield

stress - Q.V.

N.B. residual stresses from welding steel can be close to the yield point.N.B. corrosion products can induce large stresses by “wedging”.

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N.B. small-radius notch tip and even smaller-radius crack tip are STRESS RAISERS

A “wedging action” by corrosion products of 10 ksi (10,000 psi) can induce 300 ksi (300,000 psi) at the crack tip.

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Corrosion product wedging “denting” of S.G. tubes in some PWRs . . .

Boiling in crevice concentrates impurities - can lead to acid + Cl- at seawater-cooled sites.

“Hour-glassing” of Alloy-600 tubes led to severe straining and cracking of tubes. Surrey PWR in U.S. was first to replace S.Gs., because of denting.

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Time to Failure

Major damage during SCC occurs in late stages as cracks progress, cross-sectional area decreases, stress increases until final failure occurs by mechanical rupture.

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Environmental Factors

No general pattern, SCC common in aqueous solutions, liquid metals; also found in fused salts, nonaqueous inorganic liquids . . .

N.B. Coriou (France) cracked Inconel-600 in pure water at 300C in 1959!!!

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Environments that may cause stress corrosion of metals and alloys

Material Environment Material Environment

Aluminum alloys NaCl-H2O2 solutions Ordinary steels NaOH solutions NaCl solutions NaOH-Na2SiO2 solutions Seawater Ca, NH3, and NaNO3

Air, Water vapor solutionsCopper alloys NH3 (g & aq) Mixed acids (H2SO4-HNO3) Amines HCN solutions

Water, Water vapor Acidic H2S solutionsGold alloys FeCl3 solutions Seawater

Acetic-acid-salt solutions Molten Na-Pb alloysInconel Caustic soda solutions Stainless steels Acid chloride solutionsLead Lead acetate solutions such as MgCl2 and BaCl2

Magnesium alloys NaCl-K2CrO4 solutions NaCl-H2O2 solutions Rural and coastal Seawater atmospheres H2S Distilled water NaOH-H2S solutions

Monel Fused caustic soda Condensing steam from Hydrofluoric acid chloride waters Hydrofluosilicic acid Titanium alloys Red fuming HNO3, N2O4,

Nickel Fused caustic soda seawater, methanol-HCI

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Increasing temperature accelerates SCC:

Most susceptible alloys crack 100C; Mg alloys crack at room temperature.Alternate wetting and drying may aggravate SCC - accelerate crack growth (possibly because of increasing concentration of corrosive component as dryness is approached).

Effect of temperature on time for crack initiation in types 316 and 347 stainless steels in water containing 875 ppm NaCl.

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Some Data for Recommending Service of CS or Ni Alloy in CausticNACE caustic soda chart super-imposed over the data on which it is based.

Area A:Carbon steel, no stress relief necessary; stress relieve welded steam-traced lines;

Area B:Carbon steel; stress relieve welds and bends;

Area C:Application of nickel alloys to be considered in this area; nickel alloy trim for valves in areas B and C.

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Metallurgical Factors in IGSCC

In austenitic SS and Ni alloys, sensitization is of major importance in determining susceptibility to IGSCC . . . depletion of grain boundaries in Cr because of carbide precipitation makes them vulnerable to attack. e.g., IGSCC of recirculation piping in BWRs (type 304 SS) induced by 200 ppb dissolved oxygen in the otherwise pure H2O coolant resulted in a major replacement problem. Plants using L-grade experienced very much less SCC.

Al alloys (e.g., with Mg and Zn) are also susceptible to IGSCC because of precipitation within grain boundaries . . . Mg-rich precipitates can denude the grain boundaries of Mg, make them susceptible to attack in aqueous media.

N.B. In grain-boundary-precipitate mechanisms for inducing IGSCC, very local galvanic effects between precipitates and matrix are important:• some precipitates are ANODIC;• some precipitates are CATHODIC.

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Grain boundary segregation of alloy constituents or impurities (withoutprecipitation of separate phases) can also induce IGSCC.

e.g., Mg enrichment of grain boundaries in Al alloys is a factor in IGSCC- promotes local dissolution and hydrogen entry (maybe to form hydride,

MgH);

- also . . . grain boundary enrichment of impurities and/or C in Fe-base alloys, Ni-base alloys and austenitic stainless steels can contribute to IGSCC;

- segregation of P, Si, S, N, B reported; only clear link with IGSCC reported for P in austenitic SS in oxidizing aqueous solutions, for P in ferritic alloys in nitrate and caustic solutions.

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Transgranular SCC

Lattice structure in metal/alloy matrix important: dislocation emergence, movement along slip planes under stress, and similar factors that can disrupt passivating films, will promote dissolution of metal at highly localized and strained areas.

Irradiation-Assisted SCC (IASCC)

Since 1987, some in-reactor components have cracked in LWRs . . generally in core-support structures at the top of the vessel (austenitic SS, Ni alloys). More widespread in BWRs than PWRs . . . radiolytic chemical species (especially oxidizing radicals) seem to be the cause.

IASCC of Alloy-600 (Inconel) penetrations in several PWR vessel heads have led to leaks and boric-acid corrosion of RPV head steel (e.g., Davis Besse). Heads replaced.

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Mechanism of SCC SCC is very complex; probably no single mechanism, but several operating at the same time. Models (scientific descriptions) of mechanisms of two types:• dissolution;• mechanical fracture.

Dissolution Models of Crack Propagation Major model is based on Film Rupture . . . (“slip-dissolution”) . . . high stresses at crack tip create local area of plastic deformation - ruptures passive films, exposed metal dissolves rapidly . . . some say periodic dissolution and re-passivation, some say crack tip always bare.

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periodic rupture

Schematic representation of crack propagation by the film rupture model.

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Mechanical Fracture Models of Crack PropagationCorrosion Tunnel;

Corrosion tunnel models.

(a) Schematic of tunnel model showing the initiation of a crack by the formation of corrosion tunnels at slip steps and ductile deformation and fracture of the remaining ligaments.

(b) Schematic diagram of the tunnel mechanism of SSC and flat slot formation.

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• Adsorption of impurities at the crack tip promotes the nucleation of dislocations; – lead to shear-like fracture (seemingly brittle).

• Tarnish Rupture;Cracks propagate by alternate film growth and (brittle) film fracture, followed by rapid film formation over exposed metal.• Film-Induced Cleavage;

– thin film forms;– brittle crack initiates in layer;– crack moves from film into matrix;– crack continues through ductile matrix until it blunts and stops;– process repeats.

• Adsorption-Induced Brittle Fracture;Species adsorbing at crack tip alter inter-atomic bond strengths, lower stress required for fracture; propagation should be continuous.• Hydrogen Embrittlement;Cathodic processes involving hydrogen-ion reduction can inject H into matrix . . . this can embrittle metal, promote cracking . . . most likely in ferritic steels but also possible in Ni-base, Ti and Al alloys (contributes to SCC of carbon steel feeders at Point Lepreau …?).

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Prevention of SCC

1. Lowering the stress below the threshold value if one exists. This may be done by annealing in the case of residual stresses, thickening the section, or reducing the load. Plain carbon steels may be stress-relief annealed at 590 to 650C, and the austenitic stainless steels are frequently stress-relieved at temperatures ranging from 820 to 930C.

2. Eliminating the critical environmental species by, for example, de-gasification, demineralization, or distillation.

3. Changing the alloy is one possible recourse if neither the environment nor stress can be changed. For example, it is common practice to use Inconel (raising the nickel content) when type 304 stainless steel is not satisfactory. Although carbon steel is less resistant to general corrosion, it is more resistant to stress-corrosion cracking than are the stainless steels. Thus, under conditions which tend to produce stress-corrosion cracking, carbon steels are often found to be more satisfactory than the stainless steels. For example, heat exchangers used in contact with seawater or brackish waters are often constructed of ordinary mild steel.

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4. Applying cathodic protection to the structure with an external power supply or consumable anodes. Cathodic protection should only be used to protect installations where it is positively known that stress-corrosion cracking is the cause of fracture, since hydrogen embrittlement effects are accelerated by impressed cathodic currents.

5. Adding inhibitors to the system if feasible. Phosphates and other inorganic and organic corrosion inhibitors have been used successfully to reduce stress-corrosion cracking effects in mildly corrosive media. As in all inhibitor applications, sufficient inhibitor should be added to prevent the possibility of localized corrosion and pitting.

6. Coatings are sometimes used, and they depend on keeping the environment away from the metal - for example, coating vessels and pipes that are covered with insulation. In general, however, this procedure may be risky for bare metal.

7. Shot-peening (also known as shot-blasting) produces residual compressive stresses in the surface of the metal. Very substantial improvement in resistance to stress corrosion found as a result of peening with glass beads. Type 410 stainless was exposed to 3% NaCl at room temperature; type 304 to 42% MgCI2 at 150C; and aluminum alloy 7075-T6 to a water solution of K2Cr2O7-CrO3-NaCl at room temperature.

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Corrosion Fatigue The fatigue fracture of a metal aggravated by a corrosive environment or the stress corrosion cracking of a metal aggravated by cyclic stress.

N.B. Fatigue fracture usually occurs at stresses below the yield point but after many cyclic applications of the stress.Typical “S-N” curves:

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Fatigue-fractured material often shows most of the fracture face shiny metallic, with the final area to fracture (mechanically by brittle fracture of a reduced cross-section) having a rough crystalline appearance . . .

If corrosion-fatigue occurs, the “shiny-metallic” area might be covered with corrosion products; BUT normal fatigue fractures may also develop corrosion products - depends on environment, stress pattern, etc.

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N.B. In normal fatigue, the frequency of the stress cycles is not important. (can do accelerated fatigue tests at high frequency - the total number of cycles determines fatigue).

BUT in corrosion fatigue, low-cycle stresses are more damaging than high-frequency stresses.

Environment is important….e.g., in seawater:• Al bronzes and type 300 series SS lose 20-30% of normal fatigue resistance;• high-Cr alloys lose 60-70% resistance.

N.B. Cyclic loads mean lower allowable stresses, this must be designed into components; if there is also a corrosive environment, the allowable stresses are EVEN LOWER.

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Prevention of Corrosion Fatigue• change design so as to reduce stress and/or cycling.• reduce stress by heat treatment (for residual stress), shot peening (to change

surface residual stresses to COMPRESSIVE).• use corrosion inhibitor with care!• use coatings . . . electrodeposited

– Zn;– Cr;– Ni;– Cu;

and

– nitrided layers (heating of steels in contact with N-containing material e.g., NH3, NaCN, etc.).