Complete Removal of Rust and Mill Scale

Rust and mill scale are best removed by either pickhng or sandblasting. 

Sometimes, the final dip is a dilute solution of chronaic (30-45 g/Uter 4-6 oz/ gal) or chromic-phosphoric add, which serves to prevent rusting of the surface before the prime coat is applied. Use of chromates is less popular than formerly because their toxicity places restrictions on disposal of spent solutions. Hexava-lent chromium (Cr +) is a potent human toxin and known cancer-causing agent [7]. A relatively nontoxic pickle for steel consists of hot 3-10% ammoniated dtric acid followed by a dilute alkaline sodium nitrite solution to minimize superfidal rusting before application of paint [8]. 

Blasting. Using this procedure, scale is removed by high-velocity particles impelled by an air blast or by a high-velocity wheel. Blast materials usually consist of sand, or sometimes of steel grit, silicon carbide, alumina, refractory slag, 

The poor performance of paints on weathered steel exposed to an industrial atmosphere was shown in tests reported by Hudson [9] (Table 16.1). The relatively long life of paint shown for intact mill scale would probably not be achieved in actual service. It would, for example, be difficult to keep large areas and various shapes of mill scale from cracking before or after painting. Fragmentation of mill scale allows paint to become dislodged, particularly after electrolytic action has occurred between metal and scale by aqueous solutions that have penetrated to the metal surface. 

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