Chromate Versus Dichromate in Galvanizing

In this debate, both chromate and dichromate are galvanizing applications for metals such as steel.

Because they are both only slightly soluble in water, they are used as pigments in plating.

Both chromate and dichromate convert into trivalent chromium or Cr3+ for the tech-minded, which are salts with a distinctive colour.

Both serve as plating on steel sheets to retard corrosion and improve paint adhesion.

The Chromate-Dichromate Equilibrium

A great lab test to observe the interaction between chromate and dichromate, where say 10 drops of potassium chromate solution go into a test tube, which is shaken after adding 5 cm3 sulphuric acid.

Adding the acid produces orange-red coloured dichromate anions. Adding double the amount of sodium hydroxide (an alkaline) and shaking that up in the mix changes the solution to the yellow-coloured chromate.

What is demonstrates is that both ions exist in equilibrium and under alkaline conditions, the yellow chromate dominates, and in an acid environment, the dichromate takes the stage. So the galvanizer understands what is needed and can affect the chromium ion content toward either the reddish dichromate or the yellow chromate spectrum.

Red Lorry, Yellow Lorry - we know the kid’s rhyme, but when it comes to galvanizing metals, the yellow chromium was in vogue. We’ve all seen the traditional yellow school bus or taxi fleet. Due to the lead-containing pigment in yellow chrome, regulations have enforced changes to make it compliant.

In the end, dichromate is often added as a sealer on inorganic platings, such as zinc plating - zinc plating, as in the inorganic coating deposited on steel via external electric current. The purpose being corrosion resistance and to provide a base for paint. Chromate usually references an organic primer, which is applied to galvanized steel or zinc plating.

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