Chemical composition of St-37 type steel substrate.Elements Fe C Si Mn P S AlComposition (wt%) 99.03 0.18 0.33 0.32 0.05 0.05 0.04Therefore, researchers have made efforts to use other environmen-tally friendly chemicals in recent years. Many strategies have beenproposed to replace the hexavalent chromium with acceptablealternatives. In this regard, the trivalent chromium compounds areintroduced as passivating films on the phosphate coating. In mostrespects, this coating resembles the characteristics of hexavalentchromium.Recently, many efforts have been carried out to reduce thetemperature of phosphating process. In spite of many benefits ofthese methods, porous and non-uniform coatings precipitated onthe steel surface at low temperature which could not improve thecorrosion resistance of steel. Also, due to large crystal size of thephosphate particles formed at low temperature, they could notimprove the organic coatings’ adhesion to the steel substrate. Oneof the practical ways to overcome this problem is addition of someorganic and inorganic additives to the composition of phosphatesolution. Phosphate conversion coatings containing these additivesexhibit improved corrosion resistance and adhesion properties[17–21]. Recently, scientists’ attention has been directed to addi-tion of metal ions and inorganic additives to the phosphating bath[22–30]. However, a few efforts have been done on using organicespecially polymeric additives. One of these polymeric additivesis PVA. Polyvinyl alcohol (PVA) is a nontoxic water-soluble syn-thetic polymer which has excellent properties such as film forming,emulsifying and adhesion. It is prepared by partial or completehydrolysis of polyvinyl acetate [31–38]. Strawhecker and Manias[39] have identified the corrosion inhibitive performances of thepolyvinyl alcohol. Khairou and El-Sayed [38] studied the effect ofpolyvinyl alcohol and other polymers on the corrosion of cadmiumin a 0.5 M hydrochloric acid solution. Rajendran et al. studied thecorrosion behavior of carbon steel in the neutral aqueous solu-tion containing polyvinyl alcohol and 60 ppm of Cl−in the absenceand presence of Zn2+ions [31]. All of these experiments revealedthe proper corrosion inhibition properties of PVA on the steel sur-face. However, to the best of our knowledge, the workability of thisinhibitor in the phosphate bath has not been considered yet.In this work we have developed a room temperature zinc phos-phate coating in the presence and absence of PVA on the steelsubstrate. The morphological and the corrosion resistance of thecoatings were studied by SEM, XPS and EIS, respectively. The adhe-sion properties of the epoxy coating applied on the surface treatedsamples were studied by pull-off test. The cathodic disbonding ofthe coating was also studied.2. Experimental2.1. MaterialsSt-37 type steel specimens (80 mm × 70 mm × 2 mm) were pre-pared from FooladMobarakeh Co. with the composition given inTable 1.Table 2Chemical composition of zinc phosphate conversion coating bath without (Zn) andwith PVA additive (Zn-PVA).Composition Zn Zn-PVAPhosphoric acid 85 wt% (mL/L)11.311.3Zinc oxide (g/L) 5.0 5.0Sodium nitrite (g/L) 1.0 1.0Polyvinyl alcohol (PVA) (g/L) – 0.4Sodium hydroxide and phosphoric acid (85 wt%) were pur-chased from Mojallali Co. Zinc oxide was supplied by GoharfamCo. Sodium nitrite and polyvinyl alcohol (PVA) were prepared fromMerck Co. The chemical formula of the PVA is presented in Fig. 1.Araldite G27 7071X75 type epoxy resin and polyamide curingagent were obtained from Saman Co.2.2. Surface treatment processThe steel panels were abraded by emery paper 800 followedby acetone degreasing. The cleaned steel panels were immersed in100 cc of zinc phosphate chemical treatment bath with the compo-sition given in Table 2. The surface treatment was done at roomtemperature (25 ± 2◦C), 30 min immersion time and at pH = 3.1.Sodium hydroxide solution (5 wt%) was used to adjust the pH ofthe phosphate solution [40].After rinsing the treated specimens by distilled water and dryingin air, they were kept in a desiccator for further characterizations.2.3. Epoxy coating applicationThe epoxy resin was mixed with polyamide curing agent with70:30 w/w ratio. The solid content, epoxy value and density ofthe epoxy resin were 74–76%, 0.14–0.16 equivalent per 100 g and1.08 g cm−3, respectively. Additives, i.e. defoamer (Efka-2025) andleveling agent (BYK-306), were also added to the formulation. Thecoatings were applied on the steel sheets with and without con-version coating by a film applicator. Coatings were then cured at120◦C for 20 min. The dry thickness of the coating was 45 ± 5 m.2.4. Characterization2.4.1. Surface analysis techniquesThe surface morphology of the samples treated by zinc phos-phate conversion coating (Zn) and zinc phosphate conversioncoating containing PVA (Zn-PVA) was studied by scanning electronmicroscope (SEM) model LEO1455VP Zeiss. The composition of thecoating layers was studied by energy disper