Effect of electrodeposition conditions on the structure and mechanical properties of bronze coatings

The effect of electrodeposition conditions on the microdistribution, structure, and mechanical properties of bronze coatings was studied. The coatings were obtained from an alkaline electrolyte based on nitrilotrimethylphosphonic acid in DC, pulsed, and reverse current modes. The resulting coatings had similar morphology, regardless of the electrodeposition method. On the surface of golden-yellow bronze coatings there are smooth and luster rounded micro protrusions formed by a set of tiny particles about tens of nanometers large. Apparently, the morphological features of the coatings are due to the formation of a nano-sized film on the surface of steel and deposited alloy, which beneficially affects the joint discharge of copper and tin and prevents the contact precipitation of copper. Along with this, the uneven microdistribution of the alloy on a complex microprofile is observed, and current reversal does not have a noticeable effect on the leveling power of the electrolyte. It is assumed that application of an anodic current pulse during the electrodeposition process does not change the properties of the adsorption film formed in the presence of nitrilotrimethylphosphonic acid. A specific feature of the bronze coatings obtained is the absence of the "geometric levelling" effect in microdepressions, which can lead to the emergence of deep pores in the bronze coating during the electrodeposition process, and thus to the formation of new corrosion centers. This assumption is confirmed by the emergence of corrosion stains in the bronze coatings obtained on a rough abraded steel surface. An increased content of oxygen was found in the area of the stains, while the freshly deposited samples of bronze coatings contained no oxygen. A dense fine-grained structure and increased hardness of the electrolytic deposits of bronze and high adhesion to a flat steel substrate are accompanied by a mechanical resistance of bronze coatings to abrasion.