Tech-Tips-6---Storage-Stain-(White-Rust

NUMBER In- INDUSTRIAL 06 GALVANIZERS CORPORATION MARCH 2 0 1 3 Tech Tips

White Rust (Storage Stain) on Galvanized Coatings.

WHY GALVANIZE? For this reason, galvanizers use a chromate passivation in conjunction with their Hot dip galvanized coatings galvanizing operations to provide protection to are applied to steel to the galvanized coating during the ‘youth’ improve the anti-corrosion period of the coating. This passivation coating provides short term protection to the zinc to performance of the steel to give the stable oxides time to form on the ensure that it lasts as long as surface. possible with a minimum of White rust formation. maintenance Pure water (H2O) contains no dissolved salts or minerals and zinc will react quickly with pure water to form zinc hydroxide, a bulky white and relatively unstable oxide of zinc. Where freshly COATINGS DIFFER galvanized steel is exposed to pure water Only hot-dip galvanizing gives (rain, dew or condensation), in an oxygen deficient environment, the water will continue a coating that can reach the to react with the zinc and progressively 50 year life required of Storage stain on newly galvanized work. So-called consume the coating. The most common white rust forms when freshly galvanized steel is condition in which white rust occurs is with structural building products. exposed to water in a deficiency of oxygen. galvanized products that are nested together, One of the commonly encountered problems tightly packed, or when water can penetrate with galvanized coatings of all kinds is ‘white between the items and remain for extended periods. THICKNESS COUNTS rust’ or ‘white storage stain’. It is manifested as a bulky, white, powdery deposit that forms Avoiding white rust formation. Compared to other zinc-rich rapidly on the surface of the galvanized coat- There are a number of simple steps that can coatings, hot-dipped ing under certain specific conditions. greatly reduce or eliminate the formation of galvanizing is:- White rust can cause considerable damage to white rust. These are: the coating and is always detrimental to the  THICKER Keep the packed work dry galvanized coating’s appearance.   HARDER The surface of galvanized coatings is almost  Pack the items to permit air circulation  FULLER 100% zinc. It is the durability of the zinc that between the surfaces provides the outstanding anti-corrosion perfor-  Stack the packed items on an angle to mance for steel, yet zinc is a relatively allow water to drain off

‘reactive’ metal. It is the stable oxides that  Treat the surface with proprietary water HOT - DIP GALVANIZED form on the zinc’s surface that determine its repellent or barrier coatings to prevent durability, and these oxides are formed pro- PRODUCTS LAST moisture contact with galvanized surface. gressively as the zinc is exposed to the atmos- LONGER... phere. Carbon dioxide in particular is a con- tributor to the formation of these stable oxides. With newly galvanized steelwork, the zinc’s surface has been subjected to little oxidation

and is at its most vulnerable.

Industrial Galvanizers and its representatives have made reasonable efforts to ensure the accuracy of the information provided in this publication, but it does not constitute product advice. The information is of a general nature only and does not take into account your individual situation or needs. It should not be used, relied upon, or treated as a substitute for specific professional advice. Industrial Galvanizers recommend that you obtain your own independent professional advice before making any decision in relation to your particular requirements or circumstances. Information Technology Solutions

White Rust (Storage Stain) on Galvanized Coatings

Treating galvanized surfaces affected by TRIED & PROVEN  Wire brush the affected area to remove all white rust. white corrosion products Over 40 years of field testing Once the galvanized surface has been  Using a cloth pad wet with aluminium shows that galvanized attacked and the zinc hydroxide compounds paint, rub the surface with the pad to apply coatings perform well in have formed, it is desirable to remove the a thin film of aluminium paint to the oxide products from the surface because: harsh mining environments. affected area to blend it with the adjacent 1. their presence inhibits the formation of unaffected galvanized surfaces.

stable carbonate based oxides and Severe white rusting. Very heavy oxide 2. they are unsightly deposits and items may be stuck together. WHAT IS STORAGE The effect on the galvanized coating can range Areas under the oxidised area may be almost black of show signs of red rust. A coating STAIN (WHITE RUST)? from very minor to extremely severe and various levels of remedial treatment are thickness check will determine the extent to White rust is a post- available to deal with white rust problems at which the galvanized coating has been galvanizing phenomenon. the various levels at which they are likely to damaged. Remedial treatment to reinstate the coating should be undertaken by preparing the The presence of white rust is occur. area and applying one or two coats of Light white rusting. A light film of white not a reflection on the approved epoxy zinc-rich paint to achieve powdery residue frequently occurs on freshly required dry film thickness of 100 microns galvanized coating’s galvanized products during periods of heavy performance. rain. Provided the items are well ventilated and Chemical removal of white rust. well drained, white rust rarely progresses past Zinc hydroxide dissolves in chromic acid. this superficial stage. It can be brushed off if Bulky white rust deposits can be removed by required but will generally wash off in service washing with a 5% solution of chromic acid. WHY GALVANIZE WITH with normal weather. No remedial treatment is Care must be taken in the handling and generally required at this level. INDUSTRIAL containment of this solution and its residues. Moderate white rusting. A noticeable This treatment re-passivates the zinc surface GALVANIZERS? darkening and apparent etching of the and is well suited to treating batches of small For steel users requiring fast, galvanized coating under the affected area, parts. Other chemical treatments based on proven corrosion with the white rust formation appearing phosphoric acid can be used, however, these bulky. The galvanized coating thickness should will darken the coating in the process. protection for local or be checked to determine the extent of impact Re-passivating the galvanized surface. national projects Industrial on the coating. In the majority of cases, less This can be done by treating the surface with a Galvanizers is the than 5% of the galvanized coating will have solution of 5% sodium dichromate, 0.1% been removed and no remedial work is established hot dip sulfuric acid, brushing with a stiff bristle brush required if the appearance of the affected area for 30 seconds before thorough rinsing of the galvanizer with nationwide is acceptable once the residues are removed surface. by wire brushing. If appearance is coverage. unacceptable, the white rust affected area can be treated as follows: -

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