Alkaline Vs. Acid Cleaner

Screws require careful cleaning before plating.

Water-based cleaners are usually classed as acid or alkaline, since neutral solutions seldom make good cleaners. For certain soils an alkaline cleaner is superior, whereas for others an acid cleaner is best. Sometimes both may be required, with a water-rinse between.

Alkaline Cleaners

Alkaline cleaners are mostly soap or detergent based with pH-increasing agents added, such as sodium hydroxide or sodium carbonate, according to the desired pH range. Sodium triphosphate is good for special situations, but raises environmental issues. Alkaline cleaners should be used to remove oil and other organic soils.

Mild Acid Cleaners

Acid cleaners cannot contain soap, but instead use acid-stable surfactants (surface active agents that produce results similar to soaps) and perhaps a cellosolve (a type of organic solvent that dissolve grease and yet is soluble in water). Acetic acid and sulfamic acid supply metal-polishing characteristics. Acid cleaners are best at removing tarnish and mineral soils from copper and other metals.

Stronger Acid Cleaners

Sometimes stronger formulations are needed to remove heavy scale (a thick oxide coating) caused by heat processing. These formulations may contain nitric acid and sulfuric acid. Sometimes they contain chromic acid or oxalic acid, which is a very powerful organic acid.

Chemical Polish

Chemical polishes contain stronger ingredients that brighten or polish a metal surface, including viscous acids such as concentrated phosphoric, sulfuric and lactic acids. These may be heated to a carefully maintained temperature. Since they dissolve more metal than mild acid cleaners processing is usually carefully timed.

Electropolish

Electropolishing cleaners often contain heated lactic and phosphoric acids with a tiny percentage of one or more stronger acids in it. Electric current is applied, making the object electrically positive so a slight amount of metal is removed. Since the part must be conductive, this is a suitable method only for metals--frequently stainless steel or aluminum.

References

• Dawn Chemical Corporation: The Chemistry of Cleaning
• Delstar Metal Finishing: Electropolishing User's Guide

Resources

• University of Minnesota: Get It Plated Right--Cleaning Processes