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Guidelines for Fabricators and Installers

Recommend to your clients to wash the surface regularly

Weld Treatment - Pickling treatments after welding are a must for good performance of stainless steel. This chemical treatment removes the welding oxide and rapidly restores the passive layer which gives stainless its corrosion resistance.

Cutting Stainless Steel - an aluminum oxide cutting wheel/blade should be used, one not ever used with carbon steel.

Design, Fabrication and Handling - poor design and fabrication can lead to tea staining or more serious corrosion. For example, good designers will avoid crevices, such as intermittent welds and areas where water can collect. Competent stainless steel fabricators will avoid carbon contamination. Choose designers and fabricators experienced with stainless steel.

Installation and Inspection - after installation the complicated structure should be inspected for surface imperfections or contaminants. If discovered, imperfections should be removed and the corrosion resistance chemically restored by pickling or passivating treatments or by electropolishing. Hydrochloric acid which is sometimes used to clean cement or mortar residues should not be used on stainless steel.

Appropriate grade selection - grade 316 SS should be selected within 5 km of the coast.

Specify and insist on a smooth surface finish - to minimise the risk of discolouration, the smoother the surface finish the better. A surface roughness (Ra) of less than 0.5 micrometers is strongly recommended, A No.4 finish is not adequate. Typically , refinishing operations using 320 abrasives would achieve a finish better than a 0.5 micrometers Ra.

Components used near the sea can be made more resistant to tea staining by pickling to remove surface contaminants, electropolishing is even more effective. However, it must be noted that both processes can change the surface appearance - pickling tends to dull polished surfaces while electroplating will brighten surfaces. 316 stainless steel electro-polished is the ideal finish for coastal environments.

Chemically treat the welds - the best resistance to tea staining is achieved with a finish polished to 320 grit or finer, but linished or pickled finishes (only) may be adequate for less critical applications. Where a linished finish is desired, 320 grit (or finer) silicon carbide abrasives should be used with lubrication if possible. In these circumstances a passivation treatment (nitric acid only) should be applied to the ground/polished area. In selecting abrasives consideration should be given to matching the surrounding finish.

Where linishing is not performed, pickling of site welds (using mixed acids - hydrofluoric plus nitric) should take place as a final step in the weld procedures. This treatment will remove any fabrication contaminants and restore the passive chromium oxide layer, resulting in a corrosion resistant surface. Pickling treatments may alter bright surface finishes.

(Source of section(2): Australian Stainless Steel Development Association Technical Bulletin, No 2 July 2001)

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