Chemical composition of St-37 type steel substrate.
Elements
Fe C Si Mn P S Al Composition (wt%)
99.03 0.18 0.33 0.32 0.05 0.05 0.04
Therefore, researchers have made efforts to use other environmentally friendly chemicals in recent years. Many strategies have been proposed to replace the hexavalent chromium with acceptable alternatives. In this regard, the trivalent chromium compounds are introduced as passivating films on the phosphate coating. In most respects, this coating resembles the characteristics of hexavalent chromium. Recently, many efforts have been carried out to reduce the temperature of phosphating process. In spite of many benefits of these methods, porous and non-uniform coatings precipitated on the steel surface at low temperature which could not improve the corrosion resistance of steel. Also, due to large crystal size of the phosphate particles formed at low temperature, they could not improve 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 the polyvinyl alcohol. Khairou and El-Sayed [38] studied the effect of polyvinyl alcohol and other polymers on the corrosion of cadmium in a 0.5 M hydrochloric acid solution. Rajendran et al. studied the corrosion behavior of carbon steel in the neutral aqueous solution containing polyvinyl alcohol and 60 ppm of Cl−in the absence and presence of Zn2+ions [31]. All of these experiments revealed the proper corrosion inhibition properties of PVA on the steel surface. However, to the best of our knowledge, the work ability of this inhibitor in the phosphate bath has not been considered yet.In this work we have developed a room temperature zinc phosphate coating in the presence and absence of PVA on the steel substrate. The morphological and the corrosion resistance of thecoatings were studied by SEM, XPS and EIS, respectively. The adhesion properties of the epoxy coating applied on the surface treated samples were studied by pull-off test. The cathodic disbonding of the coating was also studied.
2. Experimental
2.1. Materials
St-37 type steel specimens (80 mm × 70 mm × 2 mm) were pre-pared from FooladMobarakeh Co. with the composition given in
Table 1.
Table 2
Chemical composition of zinc phosphate conversion coating bath without (Zn) and with PVA additive (Zn-PVA).
Composition
Zn Zn-PVA Phosphoric acid 85 wt% (mL/L) 11.311.3 Zinc oxide (g/L) 5.0 5.0 Sodium nitrite (g/L) 1.0 1.0 Polyvinyl alcohol (PVA) (g/L) – 0.4
Sodium hydroxide and phosphoric acid (85 wt%) were purchased from Mojallali Co. Zinc oxide was supplied by Goharfam Co. Sodium nitrite and polyvinyl alcohol (PVA) were prepared from Merck 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 process
The steel panels were abraded by emery paper 800 followed by acetone degreasing. The cleaned steel panels were immersed in 100 cc of zinc phosphate chemical treatment bath with the composition 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 drying in air, they were kept in a desiccator for further characterizations.
2.3. Epoxy coating application
The epoxy resin was mixed with polyamide curing agent with 70:30 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) and leveling agent (BYK-306), were also added to the formulation. The coatings were applied on the steel sheets with and without conversion 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. Characterization
2.4.1. Surface analysis techniques The surface morphology of the samples treated by zinc phosphate conversion coating (Zn) and zinc phosphate conversion coating containing PVA (Zn-PVA) was studied by scanning electron microscope (SEM) model LEO1455VP Zeiss.