Pitting Resistance Equivalent

The addition of 3%Mo improves its pitting resistance equivalent (PRE) but causes substantial changes (Fig. 10.31). The level of austenite is substantially decreased, only forming below 1134°C and never reaching more than 40% in the... 

It is also desirable for the alloy to have as high a Pitting Resistance Equivalent (PRE) value as possible and Lee (1995) calculated this number using the formula... 

Super Duplex material grades with pitting resistance equivalency (PRE) values greater than 40 may be necessary. 

Some physical properties such as coefficient of thermal expansion and modulus of elasticity of the alloys are given in Table 7.12. Other factors such as resistance to general and pitting corrosion, pitting resistance equivalent number (PREN) and the relative cost ratios are also given in Table 7.12. 

Pitting resistance equivalent number (PREN) is derived from a formula involving chromium, molybdenum and nitrogen contents of the alloys. These numbers are used as a qualitative indication of the pitting resistance of the alloys. The higher the PREN value the greater the resistance of the alloys to corrosion due to chloride. 

Small amounts of tungsten and nitrogen in the alloy decrease susceptibihty of stainless steel to pitting. Sedriks [69] introduced pitting resistance equivalent number (PREN) as an index for stainless steel ... 

Pitting resistance equivalent has influenced the pitting potential in NaCl solutions as shown in Figure 20.24. 

For a standard chromium-nickel steel with 17.0-19.0 wt% chromium and 9.0-11.5 wt% nickel, the resistance to pitting can be improved by increasing the contents of chromium and molybdenum but is not affected by higher nickel concentrations. In stainless steels, 3% chromium is equivalent to approximately 1% molybdenum. This finding led to the formulation of a pitting resistance equivalent (PRE) that is defined as (Lorenz and Medawar 1969)... 

The molybdenum (Mo) content of types 316 and 317 stainless resists the onset and development of pitting. The presence of nitrogen (N), a strong austenitic former, remarkably increases resistance to pitting and crevice corrosion. The relevant effect of Cr, Mo, and N on crevice corrosion is denoted by the widely used Pitting Resistance Equivalent Number (PREN). 

As noted in Table 3 [77], the results of multiple alloy tests in seawater are correlated to the Pitting Resistance Equivalence (PRE) number [47,48,72,77], which also correlates to alloy pterformance in FeCly. Anderson [78] and Streicher [79] used MCA in seawater tests to compare alloy performance. More recently, a simple, flat, plastic (specifically, polymethylmethacrylate, which is often referred to as "perspex ) washer has been successfully used to evaluate a series of alloys in seawater [77]. One application of the ASTM G 48 test has been in simulating leaking tube-to-tube sheet joints in seawater heat exchangers and condensers [87]. When certain highly corrosion-resistant alloys were paired in a dissimilar metal crevice (DMC) with alloys that would be expected to suffer crevice corrosion in the particular test solution, the more corrosion-resistant alloy was found to corrode due to the accelerating effects of the corrosion products from the less resistant alloy. The results of DMC tests in ferric chloride were confirmed by long-term DMC exposures in seawater [82],... 

Figure 6 demonstrates the mainly linear dependence of pitting potential on the pitting resistance equivalent in a number of stainless steels. 

Figure 6 Dependence of pitting potential Upp stainless steels In NaCI solution with c(NaCI) = 1 mol/kg at 293 K(20 C)onthe pitting resistance equivalent (PRE) [17]...

Deviations from linearity in high-alloyed molybdenum steels are due to the favourable effect of nitrogen. For these steels, the nitrogen content can be taken into account in the pitting resistance equivalent. For austenitic stainless steels alloyed with at least 3% molybdenum ... 

The modern pan metallurgic processes not only reduce the amounts of sulphur in the steel, but also have favourable influences on the inclusion and distribution of sulphides. However, in many corrosive mediums the dissolution of the sulphide inclusions results in stable pitting corrosion only in steels with a low pitting resistance equivalent. In steels with a higher pitting resistance equivalent, the corrosion sites are repassivated. 

In unavoidable, construction-related crevices and in cases where deposits must be reckoned with, under which corrosion can also occur, a material with a higher pitting resistance equivalent must be selected than the pitting resistance equivalent alone would require. 

To ensure resistance to pitting corrosion or crevice corrosion, it is therefore also necessary to select a material with a higher pitting resistance equivalent in mediums with higher temperatures and/or with higher chloride ion content levels. 

Table 6 shows examples of the pitting resistance equivalents required for steels to achieve sufficient levels of pitting resistance at various temperatures in seawater [23]. 

Table 6 Threshold temperatures for pitting resistance equivalent in seawater at a given pitting resistance equivalent [23]...

Figure 9 shows resistance to pitting corrosion of the high-alloyed steels and nickel alloys most frequently used in seawater [23]. The pitting resistance equivalent is listed for comparison [5]. 

In the ferritic chromium steels, the resistance to pitting and crevice corrosion depends on the pitting resistance equivalent PRE = % Cr -r 3.3 x % Mo. Steels with a pitting resistance equivalent of 32 are sufficiently resistant in seawater under normal conditions. Under critical conditions for crevice corrosion, materials with a pitting resistance equivalent of 35 are required [99]. 

The for the most part predpitation-free superferrite XlCrNiMoNb28-4-2 (25-4-4, 1.4575) with a nominal composition as in Table 34 and a pitting resistance equivalent of 33, also proves highly resistant to pitting corrosion [111]. 

In a superferritic steel (ELI steel) with the composition 0.022% C, 24.9% Cr, 3.63% Ni, 3.52% Mo and 0.50% Ti a critical pitting corrosion temperature of over 333 K (60 °C) was determined in a seawater-like chloride sulphate solution. A chlorine addition to the solution shifts the corrosion potential in the positive direction, reducing the pitting resistance equivalent in the critical temperature range [116]. 

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