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Sodium Bisulfate in the Metalworking Studio

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HDPE bottle of technical grade sodium bisulfate.

HDPE bottle of technical grade sodium bisulfate.

Sodium Bisulfate or “Dry Acid”

What is sodium bisulfate?

It is a solid ionic water-soluble compound formed when sulfuric acid is halfway neutralized by a sodium base such as sodium hydroxide or sodium carbonate. Sodium bisulfate is less acidic than its parent sulfuric acid and, hence, less reactive. But, for all practical purposes, it is still adequately strongly acidic for many artist metalworking tasks. Moreover, it is a dry granular solid that's much safer and easier to handle than highly corrosive liquid acid. This is where it gets the name "dry acid."

The most common and widespread use of sodium bisulfate is as a pH reducer in swimming pools, hot tubs and spas. It is sold as pH down or "dry acid" and usually contains at least 93% pure sodium bisulfate. It can also be obtained from jewelry supply houses as an acid pickle but at a significantly higher cost.

Rust and Scale Removal

In industrial settings, steel alloys are usually deoxidized or “pickled” in dilute solutions of either hydrochloric or sulfuric acid. Although the action of these acids is quick in removing surface oxidation, the toxic fumes and the dangerously corrosive nature make these acids impractical for the metal working artist studio. Hydrochloric acid vapors can travel a considerable distance and be absorbed by wood to be released late. These fumes will rust the surface of steel tools and fixtures in the workshop!

A more practical solution is a diluted solution of sodium bisulfate. There will be little to no fumes, and the chemical will be quite effective at removing rust and scale. After the solution is spent and will not dissolve more rust, it can simply be neutralized with baking soda for disposal. Or alternatively, any deposited solids can be strained and filtered, and more sodium bisulfate granules can be added to reactivate the solution. This will minimize waste, and the solids can be air dried and thrown in regular trash as they will be mostly iron oxides and hydroxides.

Pickle for Copper-Based Alloys

Sodium bisulfate is also used to remove heat scale and brazing flux from copper-based alloys in the jeweler trades. Hydrochloric acid cleans copper and brass but leaves a pink copper oxide layer behind. Sodium bisulfate, sometimes referred to as Sparex or similar, removes the pink cuprous layer as well, leaving bright metal behind. Many times sodium bisulfate solution is heated in a dedicated crock pot.
The sodium bisulfate solution acquires a blue color as more copper is dissolved. This solution can be eventually used to electroplate copper or apply an immersive copper flash patina on bare clean steel.

Anodizing Electrolyte

Anodizing aluminum is a process that grows a porous protective oxide on an aluminum substrate electrolytically. The pores allow the coating to absorb organic dyes giving transparent color to the aluminum object. The anodized part is sealed by boiling it in distilled water for 30 minutes.

Traditionally, the electrolyte of choice has been sulfuric acid in a 10-15% solution. Sulfuric acid might not be available to the studio artist in some locations. It has been discovered that sodium bisulfate solution is a suitable replacement for sulfuric acid as an anodizing electrolyte. Not only does this avoid spill and corrosion hazards, but it also saves considerably on shipping costs.

The only disadvantage of using sodium bisulfate is that a more concentrated 20-25% needs to be used for anodizing. The reason for that is there are only half the acidic hydrogen ions available to redissolve and deposit the aluminum oxide anodized coating.

Acidifier in Copper Sulfate-Based Etchant

Etching metals such as aluminum, zinc, and steel normally require an acid as a mordant. The traditional acid for etching was nitric acid. But nitric acid is extremely dangerous and exceedingly expensive to have shipped in any quantity.

In recent decades environmentally and safety-conscious etchers and printmakers have turned to metal salt mordants for etching white metals like aluminum, zinc, and steel. The main active metal salt is copper sulfate because of its relatively easy availability. This chemical has no action on aluminum on its own and only works on zinc and steel quite slowly. A source of chlorine in the form of rock salt (sodium chloride) is added to speed up the etching and make it more efficient.

Over time, the etchant pH will climb as copper ions get depleted, and the base metal is dissolved until a point is reached where basic metal salts are deposited. This hampers the etching process and makes an extra mess in the solution. By adding a small amount of sodium bisulfate as an acidulant, the pH remains low, and metal salts stay in the solution longer. When sodium bisulfate creates sulfuric acid in solution in small amounts, it does not act on aluminum, so it is not consumed.