by Rudy Sedlak
Most of the Dry acid salts are based on Sodium Bisulphate, which is half neutralized Sulfuric Acid, that conveniently can be crystallized. This is a relatively inexpensive, moderately strong acid source. It is nothing more. The "magic" in the products made from this material is in the additives. Depending on the purpose that the acid salt is being used for, their is nothing, in theory, that should prevent a sharp eyed, nimble, chemical vendor from creating an additive package that can be added to dilute sulfuric acid to duplicate this effect. As a matter of fact, scratch the "depending on the purpose", this is true across the board.
The control of acids, to make the pickling action constant is complicated in some cases, because the dissolved metals can sometimes increase the pickling activity. This is especially the case with Copper and Iron, if the bath is agitated enough to oxidize the Iron to the Ferric state. The only way to minimize this increase is to keep the etch rate down, and a high etch rate may be the very reason that the product was used in the first place. So this must be dealt with on a case by case basis, but there are ways to do this that make sense. Difficulties arise, however, when an acid pickle is used on more than one metal, then the mix of properties desired really gets out of control.
In any circumstances the vendor should be called in to analyze the spent bath to determine why the old bath does not perform as well as the the new bath. If the additives are consumed before the acid in the bath is, the bath will perform poorly even if it titrates properly. This can be resolved by changing the additive/acid ratio in the product, or getting the vendor to supply a "replenisher" that is additive rich.
All of this gets back to the fact that the vendor should be used more than they are, and in more effective ways. But this is a large area, that really deserves a separate discussion.
Dear Mr. Sedlak
I enjoyed your article about Dry Acid Salts, but you left out some clear advantages of using them over the basics acids.
These advantages include safety, environmental and shipping concerns. Sodium bisufate is much safer than liquid acids. OSHA classifies sodium bisulphate as an irritant, sulfuric acid is classified as corrosive. The NFPA (National Fire Protection Association) Hazard rating for sodium bisulphate is 1-0-1, for sulfuric acid it is 3-0-2. This corresponds to sulfuric acid having serious health effects and sodium bisulphate having slight health effects. For employee safety concerns, the product that contains sodium bisufate would be preferred.
If the products are spilled, sodium bisulphate can be swept up and put back into a container with no environmental problems to deal with. If a liquid acid is spilled it will contaminate the ground or get into the sewer or creek and cause serious environmental problems.
When shipping the two products DOT classifies sodium bisulphate as Non-regulated, with no shipping requirements. The liquid acids are classified as corrosive, therefore, they have extra shipping costs and additional packaging requirements.
These are some of the factors a finisher should consider when deciding which type of acid to use.
Sincerely,
Carl J. Knueven
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