Pitting stainless steels

The 18-8 stainless steels pit severely in fatty acids, salt brines, and salt solutions. Often the solution for such chronic behavior is to switch to plastics or glass fibers that do not pit because they are made of more inert material. 

In a few solutions such as distilled, tap, or other fresh waters, the stainless steels pit but it is of a superficial nature. In these same solutions carbon steels suffer severe attack. 

Schwenk, W., Theory of Stainless Steel Pitting , Corrosion, 20, 129 (1964)... 

Both calculations and measurements have indicated that it is possible to develop very concentrated metal chloride solutions within occluded sites. For example, stainless steel pits would be expected to contain 5 N Cl", 4 N Fe2+, 1 N Cr3+, 0.5 Ni2+, and 0.007 N Na+ and have a pFl of 0.5. The low sodium ion concentration develops as the Na+ migrates out of the crevice due to the electric field but is not replaced by any reaction in the crevice. Recent measurements (24) of active crevice sites on Type 316L stainless steel showed the following concentrations 2.9M Fe2+, 0.77 M Cr3+, 0.24 M Ni2+, and 0.06 M Mn2+. As was stated above, stainless steel will not remain passive in such a solution and can dissolve at a high rate. The autocatalytic nature of the process stabilizes the environment by exceeding the rate at which diffusion can disperse the concentrated solution. Initial dissolution rates of the order of 1 A/cm2 ( 440 in./yr) can be reached. 

Interface Potential and Pit Initiation. It is generally accepted that pit initiation occurs when the corrosion potential or potentiostatically imposed potential is above a critical value that depends on the alloy and environment. However, there is incomplete understanding as to how these factors (potential, material, and environment) relate to a mechanism, or more probably, several mechanisms, of pit initiation and, in particular, how preexisting flaws of the type previously described in the passive film on aluminum may become activated and/or when potential-driven transport processes may bring aggressive species in the environment to the flaw where they initiate local penetration. In the former case, the time for pit initiation tends to be very short compared with the initiation time on alloys such as stainless steels. Pit initiation is immediately associated with a localized anodic current passing from the metal to the environment driven by a potential difference between the metal/pit environment interface and sites supporting cathodic reactions. The latter may be either the external passive surface if it is a reasonable electron conductor or cathodic sites within the pit. 

Type 304 stainless steel pitting corrosion specimens... 

Figure 7.3 presents stainless steel pitting potential with 13% and 18% chromium as a function of molybdenum content in the presence of 1 M NaCl at 20 °C. Pitting potential increases with an increase of Cr or Mo content, resulting in a decrease of pitting corrosion [8]. Pitting analyses is used to (i) develop new aUoys, (ii) rank aUoys, (iii) perform failure... 

Inorganic inhibitors such as chromate, nitrite, nitrates, and arsenate are very efficient for ferrous alloys. They prevent stainless steel pitting corrosion by forming a monolayer or multilayer passivating films. For chromate, the film is formed by iron oxidation (anodic... 

Stainless Steel Pitting MIC Stress corrosion cracking Crevice Intergranular corrosion Concentration cell... 

Type 316 Stainless Steel Pitting Crevice Attack pitting Severe Crevice Attack... 

The presence of H+ ions and chloride content, prevents repassivation. The above process generates free acid and the pH value at the bottom of the pit is substantially lowered. It has been measured between 1.5 and 1.0. The pH value depends on the type of steel-values for stainless steel pits which are lower than for mild steel and have a pit pH of 4. This is due to the solubility product effects and depends on the presence of chromium. In stainless steels, the pH is reduced by hydrolysis of Cr + and Fe " " as well as by accumulation of chloride. 

---