Effect of shot-peening compaction process on the microstructure and corrosion behavior of an Al multiparticulate coating

Abstract To enhance the corrosion protection of a steel substrate coated with aluminum powder embedded in a sol–gel matrix, a shot-peening compaction process of the coating with sodium bicarbonate beads was developed. The initial coating exhibited a discontinuous microstructure characterized by large cracks and porosities between the Al particles throughout its depth. Microstructural, mechanical and corrosion properties were evaluated before and after shot-peening compaction. Depending on process parameters (impact velocity and coverage rate), the treatment enabled total or partial closure of the cracks and porosities, resulting in a relative density increase of up to 28%, as quantified by image analysis. A gradient in microstructure and mechanical properties was observed across the coating depth, with notable elongation of particles near the surface (average particle aspect ratio between 0.45 and 0.55), attributed to plastic deformation under impact. An increase of the particles hardness especially at the coating surface by up to 50% was measured, attributed to work hardening and grain refinement confirmed by XRD and EBSD analysis. Synchrotron XRD measurements indicated low in-plane residual stresses in the aluminum phase ( $$-10&lt;$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>10</mml:mn> <mml:mo>&lt;</mml:mo> </mml:mrow> </mml:math> $$\sigma$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>σ</mml:mi> </mml:math> &lt; 0 MPa). Salt spray corrosion tests revealed that only the fully compacted coatings achieved the target exposure time. Under optimal process parameters, a continuous aluminum layer was formed, enabling effective sacrificial protection of the underlying steel substrate via aluminum particles consumption.