Zinc is typically used for the electrodeposited coating for iron and steel when prevention of atmospheric corrosion is the primary objective. However without a subsequent treatment, typically chromate, electrodeposited zinc becomes dull and grey and will quickly develop white corrosion products after exposure to air. Chromate conversion coatings impart both an aesthetically pleasing finish as well as varying degrees of corrosion protection.
The use of Chrome VI in the conversion coatings as the passivation agent for zinc deposits are endowed with numerous advantages. These include: High corrosion resistance; Deposits which have “self- healing properties which give it good resistance to mechanical damage; Low cost due to the use of readily available and low cost raw materials along with low concentration; A wide availability of colored finishes which are aesthetically pleasing, This includes yellow iridescent, black and olive drab. However in-spite of all these inherent advantages, the toxicity of hexavalent chromium have precluded its use in modern electroplating operations.
What has become increasingly accepted in commercial use are chromate conversion coatings completely formulated with Chromium III (Trivalent Chrome) . Their development has proceeded over several stages of development over the past 20 years resulting in an optimization of the corrosion resistance performance. These developments have brought the performance of trivalent product and in some cases have exceeded the performance of hexavalent products.
Even though the development in the area of trivalent conversion coatings have been substantial, some issues must be considered during the commercial use of these products. One area that must be considered is the degree of added analytical and process control necessary in order to insure consistency of corrosion resistance can be maintained. Hexavalent conversion coatings required very little analytical or process control yet consistent corrosion resistance results were seldom an issue.
As mentioned before, trivalent conversion coatings have no potential of self- healing and therefore the handling of parts and subsequent ware environment needs to be considered. Some advances in additive chemistries to the conversion coating have addressed this issue with varying degrees of success.
Simple trivalent conversion coatings impart very thin conversion coatings with significant crack patterns in the deposit. While these can be aesthetically pleasing and blue bright, they typically provide very little corrosion protection to white corrosion products of the zinc plate. Typically 24 hours of neutral salt spray protection is given by these products.
In order to increase the corrosion protection of trivalent conversion coatings, it was found that by increasing the concentration of active ingredients, increasing the temperature of the chromate conversion coating and by the incorporation of various inhibitors into the conversion coating higher film thicknesses and subsequent increases in corrosion protection could be achieved. Salt Spray corrosion resistance of 120 hours can be achieved today in commercial operations.
Work continues today in chromate formulation with the goal of achieving both higher corrosion resistance and consistency in that performance.
It is also common practice today to use “top coats” which are applied as subsequent coatings to the trivalent chromate to enhance the corrosion resistance as well as improve consistency of these results. These materials can be formulated with inorganic material, organic type lacquers or a combination of inorganic and organic materials or hybrid products. These are typically applied in a subsequent dip and dry operation after the chromate coating.
The normally blue to clear trivalent conversion coating can subsequently be dyed to give a yellow finish which gives the appearance of a traditional yellow hexavalent conversion coating. However the yellow dye obviously does not increase the corrosion resistance which is derived from the underlying trivalent conversion coating.
Commercially acceptable black conversion coatings are available today which provide corrosion protection equal to that of hexavalent black chromates as well as a reasonably specular black coating. Typically these black conversion coatings require a post sealant to achieve the desired properties necessary.
There are no commercially available olive drab type trivalent conversion coatings. Dyed topcoats have been used for this purpose.