Stainless steels oxygen dissolved, effect

The addition of a small percentage of a noble metal to a base metal such as stainless steel or titanium can provide sites of low overvoltage for the cathodic reduction of dissolved oxygen or hydrogen ions. This permits larger currents and hence more positive potentials to be obtained at the anodic region, and promotes passivation under some circumstances . This effect has been demonstrated for stainless steels but has not been adopted in practice, since under other conditions the noble metal addition accelerates corrosion . 

The behavior along CPE varies according to the metal. The current density along CD remains fairly constant for metals such as iron, chromium and stainless steel as the potential increases the metal continues to dissolve at 100 percent efficiency. Along DE the metals dissolve in their higher valence state but at low current efficiency, since evolution of oxygen, or other anodic reaction where possible, now occurs. The film which forms on these metals is apparently very thin and readily allows the transitions of electrons from the anion to the metal by conduction or the tunnel effect. 

It may be added that the pyrotechnic manufacturer should not rely on vague notions of corrosion resistance when faced with problems such as attack by sea water, burial in the ground, etc. of completed items. Sea water is particularly detrimental even to stainless steels because of the combined optimal effect of 3-4% sodium chloride and of dissolved oxygen. Even bacteria, acting as depolarizers and activators,contribute to corrosion of steels. On the other hand, certain aluminum and magnesium alloys are resistant in varying degrees to sea water. ... 

Alloy additions that are effective in helping retain passivity in the presence of both low dissolved oxygen concentration and acid corrosion products help reduce or avoid crevice attack. Additions of molybdenum to 18-8 stainless steel (type 316) and palladium additions to titanium (see Fig. 25.2, Section 25.2) are in this category. 

The effect of the dissolved oxygen level on corrosion is dependent on the metal. For metals that form passive films like stainless steel and aluminum, a high oxygen content is favorable in that it helps to delay the initiation of pitting on... 

Niedrach, L. W., Effect of Palladium Coatings on the Corrosion Potential of Stainless Steel in High Temperatures Water Containing Dissolved Hydrogen and Oxygen, Corrosion, Vol. 162, 1991, p. 47. 

These contaminants may reduce the effectiveness of the natural oxide (passive) film that forms on stainless steels exposed to oxygen at low temperatures. If allowed to remain, these particles may corrode and produce rust-hke spots on the stainless steel. To prevent this condition, semi-finished or finished parts are given a passivation treatment. This treatment consists of cleaning and then immersing stainless steel parts in a solution of HNOj or of HNOj plus oxidizing salts. The treatment dissolves the embedded or smeared iron, restores the original corrosion-resistant surface, and maximizes the inherent corrosion resistance of the stainless steel. 

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