Pitting resistance

Syrett, B. C. and Wing, S. S., Pitting Resistance of New and Conventional Orthopaedic Implant Materials —Effect of Metallurgical Condition , Corrosion, 34A, 138-145 (1978)... 

The effects of a number of alloying elements on pitting resistance can be seen from the pitting potentials of various standard, proprietary and experimental steels given in Table 3.20. The test technique was a potentiodynamic... 

Hitzig et al. have produced a simplified model of the aluminium oxide layer(s) to explain impedance data of specimens prepared under different layer formation and sealing conditionsThe model also gives consideration to the formation of active and passive pits in the oxide layer. Shaw et al. have shown that it is possible to electrochemically incorporate molybdenum into the passive film which, as previously noted, improves the pitting resistance. 

Some duplex alloys have even better pitting resistance than type 316 and should be considered in severely pitting media. Titanium is virtually immune to chloride pitting and cupro-nickel alloys are used for condensers where sea-water is the coolant high pitting resistance in this duty is claimed for Cu-25Ni-20Cr-4-5Mo. 

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. 

Ringas and Robinson performed electrochemical tests on stainless steels and mild steels in three cultures of SRB. In all cases pitting resistance was lower in cultures of SRB. Potentiodynamic polarization... 

The choice of test method for examining pitting resistance depends on the objective. For evaluation of surface condition, the methods in the first group are preferable, while the methods in the second group give the best characterization of the alloy. 

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. 

Super dupiex stainiess steei ciassified with Pitting Resistance Equivaient (PRE) number greater than or equai to 40... 

Duplex stainless steel alloys are a mixture of ferritic (400 series) and austenitic (300 series) metals. They provide 1) resistance to stress corrosion and fatigue, 2) pitting resistance, 3) are suitable for a wide temperature range (-50°C to 280°C) and 4) are cost effective. In urea plants, duplex stainless steel is used to construct strippers, decomposers, condensers and pipe lines88. 

Two main points should be remembered from this lab. First, the breakdown potential is not necessarily the best measurement of pitting resistance. This is because pitting can occur at potentials below EM, as was demonstrated by metastable pitting in test 4. Ebi corresponds to the potential for stable pit growth and propagation only. Pits can nucleate, however, at any potential above the repassivation potential. Secondly, the effects that additional anions have on the pitting behavior is concentration dependent and not mass dependent. 

Has 21% Cr, 13% Mo, 3% W, 3% Fe, 60% Ni and suitable for use in oxidizing environments corrosion resistance better than C-276, C-4 in oxidizing media, better pitting resistance inferior to C-276, C-4 in reducing media and with respect to crevice corrosion Superior to C-22, C-276, super thermal stability attributed to the ternary system, Ni-Cr-Mo devoid of W, Cu, Ti and Ta Similar to C-276 in composition except for Cr level being 16-21% alloy is solution annealed at 1200°C and rapidly cooled to prevent precipitation of intermetallic phases thermal behavior not as good as Alloy 59 and its corrosion resistance was less than 59 1.6% Cu has been added to C59 lower corrosion resistance and thermal stability than Alloy 59... 

Increasing temperature usually also increases the pitting tendency of metals and alloys. At low temperature, high pitting potentials are observed. Temperature dependence of pitting susceptibility of stainless steels has been used in ranking of these steels with respect to their pitting resistance. 

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. 

The ability of the ferritic alloys to resist chloride see is one of their most useful features in terms of corrosion resistance. During the 1970s, developmental efforts were directed at producing ferritic materials that could also exhibit a high level of general and localized pitting resistance as well. 

Prevention—The methods outlined above to combat crevice corrosion also apply to pitting. Metals that are resistant to pitting should be used as alloying agents their passive films are more protective and more stable to halogen attack. For example, the addition of 2% molybdenum to 18-8 (type 304) stainless steel to produce 316 stainless steel significantly increases pitting resistance. 

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