Electroless plating for AZ91 of magnesium alloy

An experienced user is known as magnesium alloy is a highly corrosive material. Even if surface treated magnesium alloy -DOW treatment etc.- protection is lower than anodized aluminum alloy.Electroless nickel plating is highly corrosion protection potential. This article shows AZ91 magnesium alloy protection as electroless nickel plating.This article referred to (1).

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Preparation for electroless plating

The test panel prepared with emery paper finish and sandblasting. This testing investigates the difference between the smooth and rough surface for electroless nickel plating.

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Plating

Electroless plating performed from 1 minute to 150 minutes for measurement thickness and speed.

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Post-treatment

Nickel-plated substrates were performed with a kind of two heat treatments. One of the heat treatments is 250 deg C one hour and another one is 400 deg C one hour.

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Difference between adhesions

Adhesion evaluated with scratch testing for each heat-treated test panel. A post-treated of 250 deg C of blasted test panel was the most adherent in these test parameters. Next is 400 deg C treated test panel.

The author said this difference of adhesion is anchor effect with the blasted surface. In strictly speaking scratch testing can be affected by the hardness of plating. The hardness of 400 deg C heat-treatment is higher than 250 deg C of it. Electroless nickel plating is the crystallization -become Ni3P structure- at 400 deg C as a heat-treatment result.But the scratch test result shows adhesion of 250 deg C was higher than 400 deg C.The author said the causes of this difference was crack on the electroless nickel plating. I thought it the causes only clack but heat-treatment for AZ91 alloy. AZ91 alloy’s melting point is 533 deg C. Neer melting point can affect the crystalline structure.

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Film growth rate

The initial film growth rate has a difference between the emery paper finish and blasting. The sandblasted surface can grow faster than the emery paper finish surface. The author thinks, the initial reaction of electroless nickel plating occurs displacement plating on the magnesium alloy surface. The high active surface appears with sandblasting.

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Consideration

Magnesium alloy is affected by high-temperature heat treatment which causes decreasing mechanical property. Although, I want to see earning the same adhesion and hardness as low-temperature heat treatment.

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Reference

(1) Z.Liu. W.Gao / Surface & Coatings Technology 200 (2006) 5087-5093

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