Passivity Austenitic steels

Chloride Chloride ions are strongly adsorbed by steel, making it difficult to passivate. High chlorides in steam-water circuits increases the risk of stress corrosion cracking of austenitic steels (type 300 stainless steels) and increase the rate of pitting corrosion under sludges and deposits. 

Localized biological corrosion of stainless steels. There are three general sets of conditions under which localized biological corrosion of austenitic stainless steel occurs (Figure 6.29). These conditions should be examined for metals that show active-passive corrosion behavior. Microbiological corrosion in austenitic steel weldments has been documented. (Wahid)61, (Krysiak)14... 

A schematic summary of the alloying metals that affect the anodic polarization curve of stainless steel is shown in Fig. 4.16. The addition of 8% nickel to an alloy containing 18% chromium forms austenitic structure SS Type 304. The addition of Mn and N increases the stability of austenitic steel. The chromium content of stainless steel affects the anodic polarization curves as shown in Fig. 4.16. Nickel promotes repassivation in a corrosive environment, but concentrations higher than 30% reduces the passivation current, the critical current density, and increases the critical pitting potential. Nitrogen... 

The ferritic-austenitic steels are passive in seawater due to their high chromium content and do not suffer from general corrosion. Their resistance to pitting and crevice corrosion is raised by the molybdenum and nitrogen components. The duplex steels have therefore proved their worth well in a wide range of marine engineering apphcations [131-134]. 

These steels are passive in seawater and show practically no surface corrosion. The pitting corrosion behaviour of steels with molybdenum contents of 2-3% is also much better than in the molybdenum-free austenitic steels. Whereas pitting rates in the range of 0.65-1.58 mm/a (25.6-62.2 mpy) were measured in the molybdenum-free steel SAE 304 (X5CrNil8-10) in slowly flowing seawater, the value for the steel SAE 316 with approx. 2.5% Mo (X5CrNiMol7-12-2) is 0.03-0.34 mm/a (1.18-13.4 mpy) [150]. 

In addition to ferritic-austenitic steels (duplex steels), austenitic CrNiMo steels can also be used in waste water treatment plants. These steels are sufficiently passive in most waste waters so that uniform surface corrosion can be practically neglected however, under critical conditions, they can exhibit local corrosive attack in the form of pitting corrosion, crevice corrosion, or stress corrosion cracking. The decisive evaluation criteria for their possible application with respect to the media... 

Chromium is an essential element for forming a passive film, whereas other elements assist in stabilizing the film. The passive film is not very effective up to 10.5% chromium content, but as the chromium content is increased between 17 and 20%, the passivating film becomes very stable. Austenitic steels generally contain 18-20% chromium. Whereas the corrosion resistance of austenitic steel is increased by addition... 

It is known that the common austenitic stainless steels have sufficient corrosion resistance in sulfuric acid of lower concentrations (<20%) and higher concentrations (>70%) below a critical temperature. If with higher concentrations of sulfuric acid (>90%) a temperature of 70°C is exceeded, depending on their composition, austenitic stainless steels can exhibit more or less pronounced corrosion phenomena in which the steels can fluctuate between the active and passive state [19]. 

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