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Category: Passivation

  1. Electropolishing of stainless steels

    The process used for electropolishing is outlined, (anodic dissolution in phosphoric / sulphuric acid electrolytes). The benefits of electropolished finishes are summarised and a list of typical application industries and specific products given. Safety issues are briefly noted and the need to employ the services of competent specialists for electropolishing is stressed. Electropolished surfaces should be fully passive and no further passivation treatments are necessary.

  2. Passivation of stainless steels

    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.

  3. Practical Advice on Post Weld Treatment – Beware the Shoddy Finish

    Practical advice on the “hows” and “whys” of post weld surface treatments. Why removal of weld heat tint should be be the norm for stainless steel.

  4. Selection of stainless steels for handling citric acid (C3H4OH(COOH)3)

    Citric acid is a weak organic acid, found in fruits such as lemons, (citrus). Either the 304 or 316 stainless steel types can be considered for most storage and handling applications. Citric acid is also be used for cleaning and passivating stainless steels.

  5. Selection of stainless steels for handling nitric acid (HNO3)

    Nitric acid is strongly oxidising and promotes the resistance of stainless steel to corrosion. Generally stainless steels are resistant to corrosion in nitric acid. Nitric acid is used in the chemical ‘passivation’ of stainless steels. The 304 types are preferable to 316 types. Localised attack at grain boundaries can occur in hot concentrated nitric acid.

  6. Welding and Post Fabrication Cleaning for Construction and Architectural Applications

    This 12 page paper by Chris Baxter, Group Technical Market Support, AvestaPolarit Ltd. covers the issues of distortion during fabrication and welding, control of stainless steel welding processes and post fabrication cleaning. Buckling distortion can result from either poor design or welding practices. The key factors for good welding are clearly defined welding procedures, (BS EN 1011 and BS EN 287 and BS EN 288 are mentioned), control of heat input, temperature control and cleanliness. Welding electrode selection and methods including TIG, MMA, FCA, plasma, submerged arc and stud welding are covered. Finally, the importance of careful heat tint removal, (temper coloured areas), is discussed by either mechanical abrasion or chemical pickling cleaning methods is discussed. Although this establishes the corrosion resistance for the particular grade welded, care is needed with chemical pickling as it can change, (dull), the aesthetic appearance of the surfaces.

  7. When is stainless steel passive or active – formation of the passive layer

    The corrosion resistance of stainless steels is derived from the alloying element chromium. A chromium-rich oxide film forms naturally on the surface of the steel. If damaged, the film will normally repair itself. In this condition the steel is in the passive state. If the film is destroyed the surface is in the active state.

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