Effect of Zinc Phosphating on Corrosion Control for Carbon Steel Sheets

For many years, phosphating is one of the recognized, Improvement of carbon steel for corrosion by using phosphating method, has been made. It can augment the design process and present a number of experimentally determined factors that should be applied to present work. This work approach to protection for bare carbon steel, different factor e.g., thickness, time of phosphating adhesion force, protection potential with phosphating and protection potential with phosphating and painting. It is found that phosphating coating is proper to protect the carbon steel. The results conform to the theory of protection


Introduction
The use of phosphate coatings for protecting steel surfaces has been known since the turn of the century and during this period the greater part of the World's production of cars; refrigerators and furniture were treated this way.The first reliable record of phosphate coatings applied to prevent rusting of iron and steel is a British patent of 1869 granted to Ross.In the method used by him, red hot iron articles were plunged into the phosphoric acid to prevent them from rusting.Since then numerous developments have taken place, of which the major developments [2,3,4].
Phosphating is a conversion coating treatment largely used in many industries as a surface preparation for coating by paints and to increase corrosion resistance [1].Phosphate layer on ferrous or nonferrous metals [8][9][10][11][12] improve their surface properties by changes in the physico-chemical properties.The main applications of phosphating are for paints adhesion, to increase corrosion resistance, to oil impregnation and to promote electrical insulation [13,14].Despite of their use for corrosion protection, phosphate layers by themselves do not produce considerable improvements in the metallic substrate corrosion resistance and need to be associated with other corrosion protection methods.Phosphating processes have been developed for many years, using various additives, such as: nitrites, nitrates, chlorates, hydroxylamines, citric acids, calcium, nickel and manganese [15,16]. .

2-Chemistry of phosphating
All conventional phosphating solutions are dilute phosphoric acid based solutions of one or more alkali metal/heavy metal ions, which essentially contain free phosphoric acid and primary phosphates of the metal ions contained in the bath.When a steel panel is introduced into the phosphating solution a top chemical reaction takes place in which the iron dissolution is initiated at the micro anodes present on the substrate by the free phosphoric acid present in the bath.Hydrogen evolution occurs at the microcathodic sites.
Fe + 2H 3 PO 4 ------- Fe(H 2 PO 4 ) 2 + H 2  .…………….. (1) The formation of soluble primary ferrous phosphate leads to a concurrent local depletion of free acid concentration in the solution resulting in a rise in pH at the metal/solution interface.This change in pH alters the hydrolytic equilibrium which exists between the soluble primary phosphates and the insoluble tertiary phosphates of the heavy metal ions present in the phosphating solution, resulting in the rapid conversion and deposition of insoluble heavy metal tertiary phosphates.In a zinc phosphating bath these equilibria may be represented as: ----------

3-Kinetics of the phosphating process
Studies on the kinetics of phosphating indicate that there are four distinct stages in coating formation Fig.
(1) namely, the induction stage (a), the commencement of film growth (b), the main exponential growth stage (g) and the stage of linear increase in film growth (d).During the induction period, the oxide film remaining on the surface even after cleaning is removed.
When film growth commences, the first nuclei are formed and the rate of nucleation increases rapidly with time.This, however, depends considerably on the conditions of the surface, the pretreatment procedures adopted and the oxidizing agents present in the phosphating bath.Growth occurs in the main exponential growth stage.Addition of accelerators reduces the induction period and extends the stage of linear growth.
In the opinion of Gebhardt, the rate of phosphating depends on the rate of diffusion of Fe 2+ ions from the structural lattice to the coating/solution interface through the coating formed.Machu has found that the rate of the phosphating reaction is a function of micro anodes on the surface.
-dF A /dt = K .F A dt ……...(4) where dt .change in time; FA .surface area of anodes in micro cells; and K .reaction rate constant.The rate of formation of the phosphate coating depends primarily upon the metal viz., the ratio of anode surface which was initially present, FA , to the anode surface at any given moment, FA.The influence of various other factors controlling the rate of reaction, e.g., temperature, surface condition etc., is reflected in the value of the rate constant K, which is different for different processes [7].

4-Experimental Work 4-1 Research Directions
The main object was directed to coat with Zinc Phosphate at different operation conditions includes, time, thickness and surface preparation.The main properties which are studied like microstructures, adhesion force, protection potential with phosphating and painting.

4-2 Experimental Material
Experimental material which ,used are listed in Table 1.

4-3 Experimental Equipment
Experimental equipment which ,used are listed in Table 2 .

4-4-Working Specimen
Working specimens were of carbon steel with dimensions (6×6 cm).The metal specimens were placed in the phosphating bath at a depth of 10 cm below the solution and also 10 cm above the bottom of the solution in the stainless steel bath.The sheet specimen of carbon steel was analyzed by the specialized institution of engineering industrials.Baghdad.The results are summarized in table (3).

4-6 Bath Composition
The chemical composition of the concentrated zinc phosphating bath used in this study is shown in Table 5.

6-2-zinc phosphating
The results are listed in Table 6 out lines a brief knowledge aspect of zinc phosphating technology especially on carbon steel sheets, depending on their surface preparation, time of immersion, thickness of zinc phosphate, adhesion force and protection potential.

6-2-Surface preparation
In this work four different solutions were prepared for cleaning and etching the specimens.It was found that the more effective solution, phosphating treatment process as follow, Degreasing , pickling , rinsing , phosphating , rinsing , chromic acid sealing and drying.

6-3 Phosphating Surface Characterization
The   shown phosphating treatments provide a coating of insoluble metal phosphate crystals that adhere strongly to the base metal and provide corrosion protection as Fig ( 5) and provide a base for adhesion of paints.

6-4 Effect of structure
In Fig. ( 6) the interface of optical micrographs of zinc plated carbon steel specimens which are explore the effect of immersion time and thickness of zinc phosphating.
The above optical micrographs data highlight five points deserving some discussion: 1.The growth of a homogenous zinc phosphate this film on clean substance.
2. The effect of paint adhesion on clean substrate according to their preparation and thickness of thin film of zinc phosphate.
3. Conditions are reaches for good adhesion by providing smooth and spread surface as fig( 4).
4. The zinc phosphate coating is non-conductive so it protects the specimen's surface from electrochemical oxidation that leads to rust and corrosion.
5. The matrix of the phosphate coating form capillaries that increase the surface area and provide a mechanical interlocking structures on which the paint can adhere which explore as Figures (2,6)).

7-Conclusions
1. Zinc Phosphate, coating weighty 10-25 g/m 2 .2. Zinc phosphate coating reaction is simply known as the reaction between phosphates becomes insoluble on the surface and form a layer with metal is color.3. Application should be under control time, concentration, ph, and total acid are the parameters.4. Passivation is used at the last stage of phosphating process, chrome including 0-1-10% concentration Passivation prevent air accumulation under paint cover.60Min.

Figure ( 4
Figure (4) show that the adhesion are improved by increasing thickness of zinc phosphate because of the film of zinc phosphate is to improve wetting, and then adhesion of the point film and corrosion protection of collective surface coating protection as fig (6).

Fig
Fig.(2)shown phosphating treatments provide a coating of insoluble metal phosphate crystals that adhere strongly to the base metal and provide corrosion protection as Fig (5) and provide a base for adhesion of paints.In Fig. (6) the interface of optical micrographs of zinc plated carbon steel specimens which are explore the effect of immersion time and thickness of zinc phosphating.The above optical micrographs data highlight five points deserving some discussion:1.The growth of a homogenous zinc phosphate this film on clean substance.2.The effect of paint adhesion on clean substrate according to their preparation and thickness of thin film of zinc phosphate.3.Conditions are reaches for good adhesion by providing smooth and spread surface as fig(4).4.The zinc phosphate coating is non-conductive so it protects the specimen's surface from electrochemical oxidation that leads to rust and corrosion.

Fig ( 1 )
Fig (1) The various stages in the growth of phosphate coating (in presence of accelerator) [7]

Fig ( 4 )
Fig (4) show the relationship between immersion time and the adhesion forfive specimens.

Fig ( 3 )
Fig (3) show the relationship between immersion time and the adhesionfor five specimen.