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Rust staining can occur and has been reported as anything from a slight brown ‘bloom’ on the surface to severe surface pitting or rusty scour marks on items such as handrails. These effects are usually due to surface contamination from contact with non-stainless steel items. Iron contamination can be costly to remedy, and is avoidable. The ferroxyl test can be used to detect ‘free’ iron contamination. (108)
Corrosion risks and selection of stainless steel grades for short-term service in molten aluminium, copper, lead, tin and zinc are discussed.
Stainless steels are widely used for their good oxidation resistance at elevated temperatures. Although other forms of attack, such as sulphidation and carburisation need to be considered in certain applications, oxidation is of primary importance. Oxidation resistance is dependent, primarily, on the chromium content of the steel. The strength of the steel at the intended service temperature is also important when selecting stainless steels for high temperature service.
Stainless steels are alloys and therefore do not melt and freeze at a fixed temperature, as do metallic elements, but over a temperature range, depending on the chemical composition of the steel. Melting range does not directly affect the creep strength or oxidation resistance of individual stainless steels. A table of melting ranges for some of the common stainless steel grades is shown.
The NACE MR0175 standard for sulfide, (sulphide), stress corrosion cracking resistant metallic materials for oilfield equipment is now also available as an ISO standard 15156. This article summarises the requirements for all types of stainless steel austenitic , ferritic, duplex, martensitic and precipitation hardening stainless steels which are covered by the standard. Mill softened material will normally be found to comply with the requirements.
Guidance on methods for sorting stainless steels from low alloy and carbon steels is shown. These include physical, (colour, density, magnetic), and mechanical, (hardness), properties and chemical tests, (copper sulphate, copper chloride, nitric acid and sulphur tests). A suggested approach to a step-by-step procedure for differentiating stainless steels from carbon steels is tabulated. These methods have not been verified by the BSSA, who take no responsibility for their accuracy of the conclusions reached on steel types.
The affects of steel composition on the oxidation resistance of heat resisting stainless steels are discussed. The ferritic stainless steels can suffer strength and embrittlement problems. The austenitic types 1.4845, (310), and 1.4835, (253MA), are good all round choices for oxidation resistance. Maximum temperatures for intermittent service in dry air are lower than continuous service for austenitics. Moist air further reduces service temperatures.
Stainless steels naturally self-passivate whenever a clean surface is exposed to an environment that can provide enough oxygen. Passivation treatments are also sometimes specified for finishing stainless steel fabrications. Passivation normally involves using nitric acid. Citric acid treatments can also be considered. Recommended practices from ASTM A380, A967 and BS EN 2516 are shown.
Crevice corrosion along with stress corrosion cracking, is one of the major corrosion hazards to stainless steels. Careful selection of the steel grade can help reduce the risk of attack, but a brief review of the mechanism and factors that promote crevice corrosion should help reduce the risk of attack. Shielding corrosion beneath surface deposits works in a similar way to crevice corrosion.
Rouging is sometimes found in high purity hot water systems, usually appearing as a thin red or black powdery or ‘slimy’ deposit. The possible causes and mechanisms of rouging are described along with suggestions on how it can be avoided. Removal of rouge deposits is also discussed.