CHAPTER 3 EIGHT FORMS OF CORROSION 3-43 Mechanism • more effort and funds have been expended on stress corrosion than an all other forms of corrosion combined Two basic “models” for a general mechanism are (1) the dissolution model wherein anodic dissolution (Fig 3-79) occurs at the crack tip because strain ruptures the passive film at the tip. (2) the mechanical model, wherein specific species adsorb and interact with strained metal bonds and reduce bond strength. • The first seems more universal than the second. • For most engineering work past experience is the best guide, with reliable and valid testing second • In all cases chemistry, metallurgy, and mechanics (stress field) must be considered. 3-44 Multi environment Charts • Several charts (tables) showing cracking tendencies for metal and alloy systems in a variety of environments including liquid metals. • These tables are from Materials Technology Institute of the Chemical Process Industries, Inc. Table 3-13 shows the situation for carbon steels. • Low-alloy steels with very high strengths (i.e., AISI 4340) are much more susceptible to stress corrosion than weaker steels. • In general, susceptibility to cracking increases with strength level. Figure 3-72 Schematic diagram showing potential ranges over which SCC of carbon steels occurs in various solutions. • This means that changes in solution composition or temperature could shift the potential of the system into or out of the danger zone. Cathodic protection and/or inhibitor additions can inhibit cracking. Table 3-14 Environments vs. Various wrought stainless steels Table 3-15 Environments vs. Copper alloys Table 3-15 Environments vs. Aluminium alloys • Table 3-17, Zirconium and its alloys are resistant to stress corrosion in pure water, moist, air, steam, and many solutions of sulfates and nitrates. • It could crack in FeCl3 and CuCl2 solutions, halogens in water, halogen vapors, and organic liquids such as carbon tetrachloride, and fused salts at high temperatures • Columbium (niobium) and tantalum are not subject to usual stress corrosion. They can be embrittled by hydrogen. This embrittlement of tantalum in hot acids can be inhibited by contact with platinum. • Magnesium alloys are being used in many cases where light weight is an important factor, contrary to the general impression of the poor corrosion resistance of magnesium. • Alloys containing manganese have good resistance, but those with high aluminum or zinc content are quite susceptible to stress corrosion. 3.45 Classification of Mechanisms. • The complexity of the interactions between various environments, nature of the alloy, metallurgical structure, etc., indicates the impossibility of one unified mechanism for stress corrosion of all metal-environment systems. • M.A. streicher classified some SCC mechanisms that may be operative in different systems as follows : a. Dislocation coplanarity. Resistance to cracking corresponds to the dislocation pattern. - stainless steels = planar arrays -resistance alloys = cellular or tangled b. Stress-aging and microsegregation - stress aging of austenitic stainless steels jerky plastic flow. This phenomenon is associated with microsegregation of solute atoms to dynamic defects in the crystal structure. c. Adsorption. Surface active species adsorb and interact with strained bonds at the crack tip, causing reduction in bond strength and leading to cracking propagation. 2. Dissolution Mechanisms. a. Stress-accelerated dissolution. Crack propagates by localized anodic dissolution. Principal role of plastic deformation is to accelerate the dissolution process. 2. Dissolution Mechanisms. b. Film-Formation at Cracking Wall. As the crack progresses, the film on the crack walls is repaired and serves as a cathodic site. 2. Dissolution Mechanisms. c. Noble element enrichment. The copmosition of the slip step has a lower nickel concentration than that to the enriched surface; and the slip step dissolved until the nickel is enriched to the same composition as the preexisting surface. 2. Dissolution Mechanisms. d. Film rupture. Stress-corrosion cracking proceeds by successively breaking a passive film. At the point of rupture, dissolution proceeds until repassivation occurs. 2. Dissolution Mechanisms. e. Chloride ion migration. The chloride ion migrates through the cracked film toward the region of highest stress. The chloride ion then acts to break down the film, Thereby allowing metal dissolution. 3. Hydrogen Mechanisms a. Hydride formation. Hydrogen enters type 304 stainless steel to from martensite, which will diffuse into stringers normal to the direction of stress and the cause cracking. 3. Hydrogen Mechanisms b. Hydrogen embrittlement. Hydrogen accumulates within the metal at the crack tip, leading to localized weakening, either by void formation or lowering the cohesive strength. Cracks propagate by mechanical fracture of the weakened region. 4. Mechanical Mechanisms a. Tunnel Pitting and Tearing. Crack propagates by formation of deep pits or tunnels via dissolution followed by lingking of these pits or tunnels by ductile rupture. 4. Mechanical Mechanisms b. Corrosion Product Wedging. The corrosion products build up in existing cracks and then exert a wedging action. 3.46 Methods of Prevention 6. Coatings - Keeping the environment away from the metal. 7. Shot-peening (also known as shot- blasting) produces residual compressive stresses in the surface of the metal. Woelful and Muhall show very substantial improvement in resistance to stress corrosion as a result of peening with glass beads.
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