Surface modification and finishing of ferrous and nonferrous metals is widely carried out using a metal pretreatment technique known as phosphating. Phosphating is favored for reasons of providing good corrosion resistance, wear resistance, adhesion and lubrication properties, high speed of operation and low cost. The process of phosphating is widely used in automobile, process and appliance industries. Most of the phosphating processes are carried out at high temperatures typically around 95 degrees C, which directly consumes a lot of energy when used on a large scale. Other problems associated with phosphating are frequent scaling formed on heating coils and the overheating of the bath solution. This overheating causes an increase in free acidity in the bath, leading to delayed precipitation effect of the phosphate coating. In simple terms it delays the entire process.

The drawback of high energy involved has been overcome by choosing a low-temperature phosphating process, which is comparatively slow. However the low-temperature phosphating process is usually accelerated by chemical, mechanical and electrochemical methods. Electrochemical acceleration might be either anodic or cathodic in nature. In the current scenario there is a need for information relating to the mechanism of formation and the characteristics of the coating formed by the electrochemical processes.

Researcher Sankara Narayanan and his team at National Metallurgical Laboratory, Madras Centre, Chennai (India) have developed zinc-zinc phosphate composite coatings on mild steel substrates by employing cathodic phosphating process. After suitable pretreatment substrates were dipped in a phosphating solution (zinc oxide [ZnO] 2.04 g/l, phosphoric acid [H3PO4, 85%] 16 ml/l and sodium hydroxide [NaOH] 6.7 g/l) maintained at a temperature of 26 degrees C to 28 degrees C. Graphite discs were used as counter electrodes. The operating conditions used were a pH of 2.9, current density from 4 mA/cm2 to 6 mA/cm2 and a free acid to total acid ratio of 1:7.82. Deposition of the zinc phosphate coating was done in galvanostatic conditions. The duration of the coating was 60 min. Coated substrates were rinsed with deionised water and then dried using compressed air.

For evaluating the corrosion resistance of the zinc phosphate-coated substrates, they were subjected to a series of tests such as immersion test (3.5% sodium chloride solution for 24 h), galvanic corrosion test, salt spray test and potentiometric polarisation and electrochemical impedance spectroscopy. The research team explained the mechanism of coating to be taken in three steps. First a layer of metallic Zn is deposited on the surface of the substrate with hydrogen evolving simultaneously. This makes available hydrogen ions at the metal solution interface causing a progressive increase in the pH at the interface, which is conducive in converting soluble primary phosphate to an insoluble tertiary phosphate. This causes a layer of zinc phosphate to be deposited over the thin Zn layer. Finally due to simultaneous metallic Zn deposition and hydrogen evolution, zinc phosphate is deposited in the adjacent areas. The process continues till there is any available metallic Zn available on the surface. The corrosion resistance was evident when the decrease in weight due to corrosion was monitored on the zinc-coated and uncoated mild steel substrates.

The corrosion resistance of the coating is increased due to formation of nonmetallic zinc corrosion products (ZnO and zinc hydroxychloride). A more notable feature is that the increase in corrosion resistance is directly dependent on the immersion time of the substrate. In conclusion the corrosion resistance of the zinc-zinc phosphate composite coatings produced by cathodic phosphating act as a mechanical barrier and protect against corrosion for a longer period of time as evident from the stability exhibited by the coatings over a duration of one week.

"We are involved in some modification of the process and it will be patented after the modifications have been implemented," Narayanan tells Technical Insights.

Details:

T. S. N. Sankara Narayanan

National Metallurgical Laboratory

Madras Centre, CSIR Madras Complex, Taramani, Chennai 600 113

Phone: +91-044-2254-2077, +91-044-2254-0104

Fax: +91-044-2254-1027

E-mail: tsnsn@rediffmail.com

To comment on this article, write to us at tiresearch@frost.com

To find out more about Technical Insights and our Alerts, subscriptions and research services, access http://ti.frost.com