Deposition of Al-Zn-Mg Alloy Coating on an A36 Steel Substrate: Evaluation of Microstructure, Porosity and Adherence

Al and Zn-based metallic coatings are widely used to protect a significant diversity of steel components and structures from corrosion. The quality of coatings depends largely on their properties and spraying conditions. The purpose of this work was to find the optimal thermal spray parameters to manufacture Al-4.2%Zn-2%Mg alloy coatings on an A 36 steel substrate from diagnostic tests of porosity, adherence, and microstructure. Furthermore, adhesion of the coating onto the substrate after the precipitation heat treatment was investigated. Microstructural characterization was performed by scanning electron microscopy (SEM), the porosity was determined by Archimedes principle and the adhesion was evaluated by tensile testing. The optimum thermal spray distance, maintaining the thickness of the coating constant at 300 μm, was 0.20 m, in which the lowest porosity percent (9%) and the highest adherence (7.16 MPa) were attained. The coating increase of thickness and the post-heat treatment precipitation caused an adhesion decrease. However, heat-treated coatings with thicknesses between 150 and 300 µm could be used for cathodic protection of A36 steel because their adherence values (7.33 and 6.69 MPa, respectively) meet those required by the flame thermal spraying process.