Optimization of the formation technology of tripolyphosphate coating on mild steel

Tripolyphosphate conversion coatings are promising due to the active type of anti-corrosion protection. However, to be introduced into production, it is necessary to optimize the technology of tripolyphosphate coating deposition. Coatings were deposited to samples of st05kp cold-rolled sheet steel (analogs G10050, G10060, 1CR, 2CR, D6-2, DG-2) from aqueous solutions of sodium tripolyphosphate (4 %, 6 %, 10 %, 12 %, 14 %) at t=80 °C by dip coating and sputtering. The specific weight and morphology of the coating were determined. The corrosion-protective capability was studied in the G-4 climatic chamber at 90 °C and 100 % humidity using Akimov's test. The prospects of the dip coating and sputtering methods were shown. It was revealed that in the dip coating method, the specific weight of the coating was 1–4 g/m2 and increased linearly at a rate of 0.3–0.35 g/m2 by 1 % (wt.) Na5P3O10. For the sputtering coating method, it was revealed that at 4–8 % Na5P3O10, the growth rate of the specific weight was 0.2 g/m2 by 1 % Na5P3O10 and the specific weight exceeded that of the coating obtained by the dip coating method, due to accelerated oxygen access and increased coating formation rate. At 10–14 % Na5P3O10, the growth rate of the specific weight was 0.55–0.65 g/m2 by 1 % Na5P3O10. However, the specific weight was lower than that of the coating obtained by the dip coating method, due to the self-compaction of the iron tripolyphosphate matrix and decreased mass of the Na5P3O10 filler. Using accelerated corrosion testing methods, the optimum Na5P3O10 concentration to obtain a coating with the highest corrosion-protective capability was 6 %–10 % (wt.). The correlation of the protective capability of the coating samples with the coating defects and cracks was revealed