Electroless Ni–P deposition plus zinc phosphate coating on AZ91D magnesium alloy

An electroless Ni–P deposition process has been developed to treat the AZ91D magnesium alloy surfaces against corrosion. Magnesium alloy AZ91D was first phosphatized in a zinc phosphating bath containing molybdate. Then an electroless Ni–P deposition was carried out on the phosphate coating from a sulfate solution. The phases in the phosphate coatings were analyzed by XRD. Microstructures of phosphate coatings and electroless Ni–P depositions were observed by SEM and EDS. It was found that there was metallic zinc in the phosphate coating and the addition of Na2MoO4 in the phosphating bath resulted in the increase of zinc in the coating. A lot of disperse metallic zinc particles acted as the catalyst nuclei for the succeeding Ni–P deposition. Consequently, the Ni–P depositions with dense and fine microstructure were obtained on the phosphate coatings gained from the phosphating bath wherein 2.0∼2.5g/L Na2MoO4 was added. The Ni–P plus phosphate coatings on the AZ91D magnesium alloy exhibited acceptable corrosion resistance as shown by the results of the Salt Spray Corrosion Test.