Alkaline Cleaning for substrate
Alkaline cleaners are the most common method of soil removal for metal preparation prior to the application of powder coatings. Cleaners based on sodium hydroxide (caustic) are very economical where cleaning by saponification is desirable. Caustic cleaning media are highly reactive on non-ferrous surfaces and they can cause over-etching on aluminum and zinc surfaces, possibly creating smut and adding zinc to the solution. Caustic cleaning residues are also difficult to rinse away, especially if the solution temperature is in the high range.
Alkali silicates are excellent for cleaners that are used on non-ferrous surfaces. Silicates can provide good cleaning with minimal chemical attack and they do a good job of soil emulsification. They are a little more costly than alkalis and not easily rinsed.
Synthetic detergents and surfactants offer many variations in composition. In some cases they cost a little more than alkalis but they provide a longer bath life which offsets the higher raw material cost. With good performance, easier handling and disposal, and superior effectiveness over a wider array of metals, these products are a good solution for many systems.
Typically, a mild alkaline cleaner (pH of 9 to 10) will provide better soil removal and longer bath life than a high caustic solution. Residues of alkali salts will kill the free acid, drop out metal salts and kill the phosphate bath. A mild alkaline cleaner, prior to the phosphate stage, will aid in the formation of a more uniform, dense phosphate coating, leading to better paint adhesion and corrosion protection.