Crack initiation electrochemical corrosion

Electrochemical noise A variety of related techniques are now available to monitor localized corrosion. No external polarization of the corroding metal is required, but the electrical noise on the corrosion potential of the metal is monitored and analyzed. Signatures characteristic of pit initiation, crevice corrosion and some forms of stress corrosion cracking is obtained. 

Electrochemical aspects of the stress-corrosion behaviour have been investigated, mainly in neutral solutions. The open-circuit potential of Ti-8Al-lMo-l V is —800mV (v5. S.C.E.). The crack initiation load reaches... 

Depending on the corrosion and metal surface reactions, different types of corrosion can occur under different conditions (Fig. 1-1). The types of common electrochemical corrosion are usually subdivided into corrosion without mechanical stress and corrosion with mechanical stress. Pitting corrosion or selective corrosion can sometimes initiate other types of corrosion damages, e.g. stress-corrosion cracking or corrosion fatigue. The various types are discussed in the following section. 

In the first part of this ch iter, crack initiation mechanisms are reviewed and quantitative approaches are discussed, bearing in mind the limitations related to the evaluation of localized corrosion-deformation interactions. A particularly interesting example of electrochemical and mechanical coupling effects during CF in duplex stainless steels will illustrate the complexity of the interactions that lead to crack initiation. 

The electrochemical approach presented here has many limitations. First of all, the kinetics of the electrochemical reactions are closely dependent on the cyclic plasticity and the number of cycles [9,10]. Thus, predictive laws are very complex. Moreover, these laws use the local current densities, which are very difficult to model. Finally, this approach does not really take into account the local corrosion-deformation interactions (GDIs) and the effects of the corrosive solution on the deformation mode. Indeed, such synergetic effects between corrosion and deformation can be of prime importance the following examples emphasize the role of GDI in crack initiation processes. 

Localized corrosion and stress corrosion may often be observed. Stress corrosion cracks usually initiate at pits in many systems. The role of pitting is to disrupt films that otherwise prevent the ingress of hydrogen (118, 119). Electrochemical polarization technique may be used to distinguish between SCC and HE mechanisms in high-strength steels in sodium chloride solutions (120). 

H.S. Isaacs, Initiation of stress corrosion cracking of sensitized type 304 stainless steel in dilute thiosulfate solution,. Electrochem. Soc. 135 (1988) 2180-2183. 

Since stress at a surface is highest at the root of a notch or imperfection, it is there that a crack grows most rapidly. Surface defects acting as effective stress raisers include corrosion pits and fatigue cracks. Additionally, at surface defects that are sufficiently deep, the electrochemical potential differs from that at the surface also, the composition and pH of solution within the defect can be altered by electrochemical transport through operation of differential aeration cells. TTiese changes plus increased local stress sometimes combine to initiate S.C.C. as well as accelerate crack growth [50]. 

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