Corrosion features stress

The effect of environmental variables upon the logarithm of velocity V5. K relationship has been examined for a few alloys in some conditions of heat treatment. While it cannot be certain that similar results would be obtained with all alloys, the results reported do show interesting features that may have points in common with all alloys. For an Al-Zn-Mg-Cu alloy (7075-T651) the stress-corrosion plateau velocity was a maximum in 5 m KI solution under potentiostatic conditions at -520 mV (v5. S.C.E.), reaching about 2 X 10 to 5 X 10 cm/s, whereas in 3% NaCl under open-circuit... 

Undue static or cyclic stressing and other features which give rise to stress concentrations should be avoided as these may lead to premature failure by stress-corrosion cracking or corrosion fatigue. 

Ensure stress-raising features such as holes, welds, edges, rapid changes in section, etc, are minimised in their exposure to the stress-corrosion cracking risk environment. Heat treatment of components may give beneficial results. 

A feature of corrosion studies which has been stressed recently (2) is the complete failure of laboratory tests on their own to predict how reliable operation of some nuclear steam generators can be maintained. At least a part of this problem is likely to arise from different redox and/or pH conditions imposed by the solution in autoclave tests and in plant conditions and many low level contaminants could be involved. In view of what has been said earlier concerning the role of Mo(VI) in stagnant water it is clear that some data, at least on the thermodynamics of aqueous Mo species, should be sought at high temperatures. 

Adhesives and sealers can be an important part of a total corrosion protection system. Structural bonding procedures and adhesives for aluminum, polymer composites, and titanium are well established in the aerospace industry. Structural bonding of steel is gaining increasing prominence in the appliance and automotive industries. The durability of adhesive bonds has been discussed by a number of authors (see, e.g., 85). The effects of aggressive environments on adhesive bonds are of particular concern. Minford ( ) has presented a comparative evaluation of aluminum joints in salt water exposure Smith ( ) has discussed steel-epoxy bond endurance under hydrothermal stress Drain et al. (8 ) and Dodiuk et al. (8 ) have presented results on the effects of water on performance of various adhesive/substrate combinations. In this volume, the durability of adhesive bonds in the presence of water and in corrosive environments is discussed by Matienzo et al., Gosselin, and Holubka et al. The effects of aggressive environments on adhesively bonded steel structures have a number of features in common with their effects on coated steel, but the mechanical requirements placed on adhesive bonds add an additional level of complication. 

Silverman has defined a number of useful expressions that allow one to utilize the rotating cylinder method with a variety of practical geometries (12,15). Both shear stresses and mass transfer coefficients are included in the derivations described (12). Table 1 in NACE standard TM-0270-72 summarized the various features of experimental systems for studying flow induced corrosion (22). 

Environmentally induced cracking consists of (i) stress corrosion cracking (ii) corrosion fatigue and (iii) hydrogen-induced cracking. The general features of these modes of failure are given below ... 

When one talks about reversibility of the Rehbinder effect, the presence of a thermodynamically stable interface between mutually saturated solid phase and the liquid, as well as complete disappearance of these effects upon the removal of the medium (e.g. by evaporation) are implied. These features emphasize principal difference between the Rehbinder effect and corrosive action of the medium. At the same time, one has to realize that it is not possible to draw here a distinct border line. The term disintegration covers a broad range of processes from idealized cases of purely mechanical breaking to destruction by corrosion or dissolution. The Rehbinder effect, i.e. the lowering of strength due to adsorption and chemisorption, stress-caused corrosion, and corrosion fatigue, occupies some intermediate place between these extremes. All these phenomena represent a certain degree of combination between the mechanical work performed by external forces and chemical (physico-chemical) interaction with the medium. 

The necessary conditions for the occurrence of SCC are (i) a crack-promoting environment (ii) the susceptibility of the material to SCC (iii) exceeding threshold value with regard to tensile stresses. One of the distinguishing features of SCC is that stress corrosion faces suffer very low corrosion even in solutions that damage free surfaces. 

The isocorrosion curves of the steel X 5 CrNiMoCuNb 18 18 are show the stress corrosion cracking zone in Figure 20.42. The data refer to aerated sulfuric acid as the corrosive medium. A notable feature does not occur at all temperatures and concentrations, but only above a given corrosion rate. 

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