December 5, 2008

Pretreatments

Some heat scales are not rapidly soluble acids or may dissolve so slowly that attack on the basis metal occurs before the scale is removed. If this is the case, a pretreatment step can be used to change the scale so that it becomes readily soluble in dilute acids. Sometimes a good alkaline cleaning using a chelated alkaline derusting compound is beneficial.

Posselt et al. found that a caustic soda permanganate pretreatment made many types of heat scales easy to remove in regular acid pickling baths.

Fore metals that form difficult to remove scales, molten salt are often used. The du Pont sodium hydroxide bath containing 1.5 to 2 % sodium hydride and operating at 370 to 400 oC is an effective reducing bath. After such treatment, many scales are easily removed by a water quench and dilute acid with no attack on the basis metal. Some molten sodium hydroxide bath contain oxidizing agents such as sodium nitrate. These bath 450 to 540 oC and oxidize the scales so they can be removed by a water quench and dilute acid. Two widely known bath are Kolene bath and the Virgo bath.

Post by Aluminum Anodizing.

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