Characterisation of phosphate coatings obtained using nitric acid free phosphate solution on three steel substrates: An option to simulate TMT rebars surfaces

Phosphate coatings have been obtained on three steel substrates, (a) ferritic-pearlitic (F-P), (b) tempered martensitic (T-M) and (c) tempered martensitic containing oxide scale (T-M-O) at the top to simulate TMT (thermo mechanical treatment) rebar surfaces which are extensively used for composite concrete structure. Nitric acid free phosphate solution was used for the coating purpose. Scanning electron microscopy (SEM), glow discharge optical emission spectrometry (GDOES) and X-ray diffraction (XRD) techniques were used to characterise phosphate coatings. Acicular phosphate microstructure was obtained on T-M-O and T-M steel substrates, whereas coarser phosphate microstructure was obtained on F-P steel substrate. Thinner to thicker coatings were obtained on F-P, T-M and T-M-O steel substrates respectively. Oxide scale, on the T-M-O steel substrate promoted for deposition of phosphate compounds and thereby obtained thickest coating on T-M-O steel substrate. Zinc phosphate (hopeite) on T-M-O steel substrate and zinc phosphate (spencerite) on F-P steel substrate were detected as main phosphate respectively, whereas both zinc phosphates (hopeite and speccerite) on T-M steel substrate were detected in the same proportion. In addition to zinc phosphate, zinc iron phosphate (phosphophyllite) was detected on F-P and T-M steel substrates, whereas iron phosphate (beraunite) was detected on T-M-O steel substrate. A comparative performance against corrosion of all the phosphate coated steel substrates was evaluated by salt spray, Tafel and electrochemical impedance spectroscopy (EIS) studies. Test for extended exposure in open atmosphere as well as simulated highly humid condition were also conducted to identify coating performance. Phosphate coated T-M-O steel substrate showed 4–5 times improved resistance against corrosion than the phosphate coated F-P steel substrate. The bond strength of coated steel substrate with concrete was increased 2–26% for phosphate coating on T-M-O and T-M steel substrates while the same was decreased 4–12% for phosphate coating on F-P steel substrate.