Steels continued pitting corrosion

New alloys with improved corrosion-resistance characteristics are continually being marketed, and are aimed at solving a particular problem, e.g. improved stress-corrosion cracking resistance in the case of stainless steels improved pitting resistance or less susceptibility to welding difficulties. 

For materials like stainless steels, the mechanisms are quite different. Corrosion resistance in stainless steels is provided by a passive film that acts as a barrier between the alloy and the water. The passive film is a continuous, non-porous and insoluble film, which, if broken under normal conditions, is self-healing. Due to these characteristics, the uniform corrosion of stainless steels is usually very low and the major risk is pitting corrosion. The pitting corrosion risk of stainless steels is influenced not only ly the composition of the alloy and by water quality but also by service conditions, quality of the material and quality of the installation (fitting, soldering conditions, etc.). 

By stirring electrolyte at 350 rpm without silica particles the OCP was measured and pits were observed for X52 steel. The open circuit potential for X52 steel in sodium bicarbonate solution containing chloride content fluctuates continuously with many positive and negative peaks as shown in Fig. 2. These fluctuating peaks with both positive and negative high amplitudes occur because of pitting corrosion. 

One must be careful about the corrosion allowance for pitting corrosion of stainless steel - there should not be one. This is because pitting corrosion is not a continuing phenomenon. When pitting corrosion happens, it happens over a short time which is not the situation envisioned by Equation 2.3. 

In boiling water reactors chromium steels behave similarly to those in pressurised water reactors. 13% chromium steels withstand the effects of steam (700 °C) in continuous operation [75]. Chromium steels with 13-17% C are especially sensitive to intergranular corrosion after being welded. Also, riveted joints were seen to have pitting corrosion up to 1.7 mm in depth as well as crevice corrosion after one year [76]. 

Other tests on steels with 0-4% Cr and 0-1% A1 or Mo also showed that a steel with 2% Cr and 1% A1 represents the best compromise in resistance to uniform and local corrosion. Chromium contents above 2% continue to reduce the corrosion rates, but also increase the tendency to pitting corrosion [50]. 

Corrosion inhibitors for steels are being continuously developed because of the ubiquitous use of steel in construction and its somewhat limited corrosion resistance, especially in the presence of water. A great number of papers are on the effect of corrosion inhibitors, and the overwhelming majority deals with the effect of inhibitors on uniform corrosion. Due to environmental restrictions on common inorganic inhibitors (Freedman, 1986), several studies suggest derivatives of some amino acids as corrosion inhibitors. A survey of a number of different organic compounds commonly used as uniform corrosion inhibitors showed that most compounds hardly affect the pitting corrosion of stainless steel however, one of... 

TABLE 8.35 Influence of Different Alloying Additions and Microstructure on the Pitting and Crevice Corrosion Resistance of Duplex Stainless Steels (Continued)... 

If millscale was perfectly adherent, continuous and impermeable, it would form a good protective coating, but in practice millscale soon cracks and flakes off in places. In air, the presence of millscale on the steel may reduce the corrosion rate over comparatively short periods, but over longer periods the rate tends to rise. In water, severe pitting of the steel may occur if large amounts of millscale are present on the surface. 

---