Chrome Plating

Chroming Advantages

The hardness of chromium can reach 800-1000 HV, and is the most hard metal. It has perfect passivation performance, and is passivated quickly in the atmosphere, which results in that it’s very stable, even while in the alkali, nitric acid, sulfide, carbonate, organic acid and other corrosion mediums. It could keep luster well for a long time. However it’s soluble in hydrochloric acid and other halogen acid and hot concentrated sulfuric acid.

Chromium plating satisfactorily improves the hardness, wear resistance, light reflex and heat resistance. And it starts to be oxidated only when its temperature is higher than 500℃, and its hardness goes down only when the temperature is more than 700℃. But the chroming deposition is brittle, and easy to fall off. Normally manufacturer will deposit a layer of brass first, then a layer of nickel, finally a layer of chromium, when it’s used for steel to get satisfying corrosion resistance and appearance, because the chroming deposition is porous.

Chroming Process

In the traditional chrome plating process, the plating solution utilezes chromic acid as the base, and sulfuric acid as catalyst, and the ratio of the two is 100:1. The classic formula for conventional chromium plating solution is:

CrO3 250 g/L
H2SO4 2.5 g/L
Cr3+ 3g /L

For conventional chromium plating, only 12-15% of the electricity is used for chromium deposition, and 80-85% is used for separation of hydrogen. And Hydrogen will go into the chrome layer, also will penetrate into the basic by dozens of micrometers. Hydrogen infiltration makes the fatigue strength of steel fall by 30-40%. So the hydrogen need to be removed after plating. Before plating, the stress must be estimated, because of the high sensitivity on stress of chroming deposition. However, the plated parts have remaining stress after machining, grinding, or heating. High temperature of 150℃-230℃ will help out.

Hard chroming deposition is more thick than normal chroming, and the time is longer, usually will spend more than 10 hours.