Austenitic stainless steel crevice corrosion

In ferritic and austenitic stainless steels, crevice corrosion is almost always initiated by local activation. This can be induced in a crevice by oxidant depletion, if necessary supplemented by halides. The passivity then breaks down. The access of oxidants to the material surface, and hence the passivity, may also be hindered by local deposits. 

Stainless steels are particularly prone to crevice corrosion, and even the Fe-18Cr-8Ni-3Mo type of austenitic stainless steel, which is highly resistant to pitting when the surface is free from crevices, is susceptible although initiation of attack may take 1-2 years... 

It is also an accepted fact that the crevice corrosion ceases to grow at potentials less positive than a certain critical potential resulting in crevice protection as shown for austenitic stainless steel in Figure 22.30 [59,61]. The critical potential, Ecrev, is called crevice protection potential or the critical crevice corrosion potential. It was found for a cylindrical crevice in austenitic stainless steel that the crevice protection potential shifts in the less positive direction as a logarithmic function of solution chloride concentration [61] ... 

Fig. 7.48 Response of five austenitic stainless steels to pitting and crevice corrosion. Alloys exposed 1 month at room temperature in indicated concentrations of FeCI3 solutions. (Numbers represent weight percent.)...

Super austenitic stainless steels, such as AL-6XN and 254SMO alloys, have high resistance to localized corrosion. Chloride pitting and crevice corrosion in these alloys is very low due to the presence of high molybdenum content (>6%) and nitrogen additions. Steels with higher nickel content exhibit better resistance to stress corrosion cracking than austenitic stainless steels. 

Duplex stainless steels have a mixed microstructure of ferrite and austenite with chromium content in the range between 19% and 32%, molybdenum up to 5%, and lower nickel contents than austenitic stainless steels. They exhibit better corrosion resistance to pitting, stress corrosion cracking, and crevice corrosion than austenitic stainless steels, and are approximately twice as strong. 

C.S. Brossia, R.G. Kelly, Occluded chemistry control and the effects of alloy sulfur in the initiation of crevice corrosion of austenitic stainless steel, Corros. Sci. 40 (1998) 1851—1871. 

Y.C. Lu, M.B. Ives, Chemical treatment with cerium to improve the crevice corrosion resistance of austenitic stainless steels, Corros. Sci. 37 (1995) 145—155. 

As discussed in Section 19.2.4, stainless steels are best employed under fully aerated or oxidizing conditions, which favor the passive state. Whether used in handling chemicals or exposed to the atmosphere, the alloy surface should always be kept clean and free of surface contamination. Otherwise, crevice corrosion may cause pitting and localized rusting. Austenitic stainless steels cooled too slowly through the sensitizing temperature zones tend to rust in the atmosphere. 

Stainless steels are resistant to corrosion by most salts. The exceptions are the halide salts that cause pitting, crevice corrosion, and SCC. Of these salts, those containing chlorides are the most corrosive, followed by fluoride, bromide, and iodide salts. Stainless steels with higher chromium, molybdenum, and nitrogen concentrations will resist pitting and crevice corrosion more effectively. Austenitic stainless steels with higher molybdenum and nickel, ferritic stainless steels with no nickel or copper, and duplex stainless steels will resist SCC. 

Pitting and crevice corrosion are types of localized corrosion. This form of corrosion can be observed for instance on passive metals especially austenitic stainless steels in the presence of certain anions (chloride, bromide). Pitting corrosion (Fig. l-9b) results from a local destruction of the protective passive film, with the formation of a small corrosion anode. Pitting can start at faults in the passive film or at non-metallic inclusions like sulfides in stainless steel. Because of hydrolysis of the corrosion prod-... 

Because of its chromium, nickel, and molybdenum contents, it possesses excellent resistance to chloride-induced pitting, crevice corrosion, and stress corrosion cracking. It has better resistance than austenitic stainless steels to general corrosion in diverse conditions. Good to excellent resistance is shown to organic acids, alkalies, salts, and seawater, with good resistance shown to sulfuric, phosphoric, and nitric acids. 

While general corrosion resistance is important, one of the major reasons that nickel-based alloys are specified for many applications is their excellent resistance to localized corrosion, such as pitting, crevice corrosion, and stress corrosion cracking. In many environments, austenitic stainless steels do not exhibit general attack but suffer from significant localized attack, resulting in excessive downtime and/or expensive repair and replacement. 

Performance in the area of pitting and crevice corrosion is often measured using critical pitting temperature (CPT), critical crevice temperature (CCT), and pitting resistance equivalent number (PREN). As a general rule, the higher the PREN, the better the resistance. The PREN is determined by the chromium, molybdenum, and nitrogen contents PREN=%Cr-l-3.3 (%Mo)-l-30(%N). Table 1.4 lists the PRENs for various austenitic stainless steels. 

Chlorides in the water can cause corrosion, particularly in the case of stainless steels. Typically 304 is satisfactory up to about 200 ppm chlorides, while 316 can withstand around 1000 ppm and 4.5 percent molybdenum austenitic stainless steels and duplex stainless steels are known to have suffered from crevice attack at 2000 to 3000 ppm beneath fouling. Titanium and the six percent molybdenum stainless steels have been shown to resist crevice attack in seawater (1900 ppm chlorides) under deposits. 

TABLE 4.4 FERRITIC-AUSTENITIC STAINLESS STEEL-CABOT WROUGHT PRODUCTS (continued) Crevice-Corrosion Data in 10% Ferric Chloride at Room Temperature for 10 Days... 

Critical Crevice Corrosion Temperature for the Duplex and some Austenitic Stainless Steels In 101 FeCl3-6H20 (pHl) ... 

At least in a first stage, corrosion may not affect the whole crevice gap it has been shown that on austenitic stainless steels, dissolution is mainly located near the mouth of the crevice [3,19] (Fig. 26), whereas on ferritic stainless steels the maximum of dissolution rate is located deeper in the crevice [3],... 

Use alloys resistant to crevice corrosion, such as titanium or Inconel. Increased Mo contents (up to 4.5%) in austenitic stainless steels reduce the susceptibility to crevice corrosion. Use appropriate alloy after prescribed service tests for a specific application. 

This section deals with the corrosion behavior of duplex stainless steel, some high austenitic stainless steels, and a nickel-based alloy in 28% HCl acidizing solutions, either in inhibited or uninhibited conditions, at 130 °C. Weight loss, crevice corrosion, and stress corrosion cracking tests were carried out for 6 and 24 hours, with amine-based commercial corrosion inhibitors, originally formulated for carbon steel. 

Crevice corrosion tests revealed susceptibility to corrosion attack in the presence of corrosion inhibitors, while in their absence no preferential attack in crevice sites was observed, see, in the presence of commercial inhibitors, did not occur for any tested materials in the absence of corrosion inhibitors, transgranular microcracks were present at the bottom of pits on high austenitic stainless steel. 

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