Microstructure of phosphate conversion film formed on the surface of A36 steel prepared by micro-arc discharge plasma

Phosphate conversion film is an important surface protection film for steel. It mainly has the functions of corrosion resistance, wear resistance, plastic processing lubricity, etc., or as the basis of subsequent coating or painting. Phosphate treatment is traditionally used to form a Phosphate conversion film on the surface of steel. In this treatment method, a dilute solution of phosphoric acid and phosphate can be applied to the surface to be treated by spraying or soaking, so that it can react with the surface to form an insoluble phosphate film. This method is more difficult to control the thickness and microstructure of the formed film, and there are many pretreatment procedures. In response to future environmental policies, carbon neutrality and 2050 net zero carbon emissions must be met. In this study, the micro-arc discharge method was used from the surface pretreatment of A36 steel to the formation of the phosphoric acid film. The consistent method shortened the process steps and reduced the carbon emission of the process to comply with the environmental policy. The discharge voltage is 50 ∼ 200 V, the current is about 1.6 A, and the different process time and current are controlled, which to observe the microstructure of the phosphoric acid film and surface hardness. Possible reaction mechanisms for phosphate film formation will be discussed in this paper.


Introduction
The use of steel is one of the most widely used metals, and it is an indispensable material for industry, national defense, and agriculture.The reason why steel is widely used, that refining is relatively simple and easy to process and manufacture, and the use of different heat treatment technologies can improve the properties of steel and have an effect of upgrading [1,2].However, the disadvantage of steel materials is that both wear resistance and corrosion resistance are not very well.Although coating and electroplating methods can be used to make the surface treatment more perfect, it still cannot achieve high wear resistance and high corrosion resistance [3].ASTM A36 steel is a mild steel with a carbon content of approximately 0.25-0.29%and is commonly used in building construction, storage tanks, and strip pipes.Likewise, A36 steel is susceptible to corrosion.The corrosion mechanism is characterized by the ion exchange process between the metal and its environment, which results in physical changes and decreases in the mechanical properties.Therefore, one way to prevent early wear and corrosion is by carrying out a surface treatment process, which is, coating other metals or oxides on the surface of the metal [4].Steels are coated with anticorrosion materials, such as Galvalume, galvanized coating and organic/inorganic inhibitor coatings [4][5][6][7][8][9][10].The different between galvalume and galvanized coating, that is, Galvalume coating is a coating composed of aluminum and zinc; however, Galvanized coating is a coating that is almost 100% made of zinc or with a little addition of silicon.The disadvantage of galvanized coating is low resistance to acids and salts [4].Organic/inorganic inhibitor coating 2 processes typically use inhibitor pigments as the main component to inhibit the corrosion process.According to the inhibition mechanism, inhibitor pigments can be divided into active pigments and barrier pigments [4][5][6][7].The excellent corrosion inhibition properties are compared active pigments than barrier pigments.Although active pigments such as lead red and chromate have excellent corrosion inhibition properties, they are toxic and therefore limited by environmental protection measures.Therefore, it is necessary to develop more environmentally friendly pigments.In the other methods of corrosion protection, phosphate conversion coating is a chemical treatment with excellent binding performance due to chemical bonding between the coating and the matrix metal.Zinc, manganese and iron phosphate coatings are often used in metal coating [8][9][10][11][12].In traditional phosphating treatment, the steel workpieces are usually treated with various catalysts at high temperatures to obtain a suitable coating, which is not friendly to the environment and human health [12].At present, the main focus of the researchers is to develop accelerators to achieve environmentally friendly phosphating treatment process.Micro-arc discharge oxidation (MDO), also called plasma electrolytic oxidation (PEO), micro-plasma oxidation (MPO), anodic-spark deposition (ASD) or Micro-arc oxidation (MAO) or Micro-arc discharge plasma (MADP) that has a broad application prospect by virtue of its superiorities such as simpleness, high efficiency and eco-friendliness [13].Micro-arc discharge plasma (MADP) is a highpressure plasma electrolytic oxidation process that is widely used in the surface modification of aluminum alloys, magnesium alloys, titanium alloys and other light metals [14].The advantages of MADP combine with phosphating to form a thick, compact, and insoluble metal phosphate coating less cavities to enhance the corrosion resistance that have widely using for aluminum alloys, magnesium alloys and titanium alloys [15][16][17].However, the micro-arc discharge plasma combined with phosphating technology is rarely used in steel surface corrosion protection.In this study, the micro-arc discharge plasma combined with phosphating technology was used to A36 steel improve its surface hardness.The electrolytes were used trisodium phosphate (Na 3 PO 4 ), trisodium phosphate (Na 3 PO 4 ), mixture of trisodium phosphate (Na 3 PO 4 ) and sodium percarbonate (Na 2 CO 3 ) which with environmentally friendly properties.The influence of different electrolyte temperature, treatment time, voltage and current on the treatment effect, and discuss its feasibility on the surface treatment of A36 steel.

Experimental procedure 2.1. Material preparation
The A36 steel with a thickness of 2 mm, a length of 30 mm and a width of 20 mm was selected as the start material for the experimental workpiece.The chemical composition of A36 steel is shown in Table 1 [35,36].The surface of the workpieces were cleaned that the workpieces soak in an detergent solution then use an Ultrasonic Cleaner (Ultrasonic Cleaner, Prema, PK-B) for 10 min to remove the dirt and possible contaminants on the surface.The workpieces rinse with pure water and alcohol in next step, then use blow dry with high pressure gas.

Electrolyte solution
The electrolyte uses trisodium phosphate (Na 3 PO 4 ) and sodium percarbonate (Na 2 CO 3 ) aqueous solution.The electrolyte solution was adjusted with deionized water, and the concentrations of trisodium phosphate and sodium percarbonate were 0.067 M and 0.127 M, respectively.

Micro-arc discharge plasma
The micro-arc discharge plasma oxidation processes were carried out a programmable AC/DC power supply (Power Supplier, GW INSTEK, ASR-3200).The micro-arc discharge plasma coatings were prepared under a constant cathode current of 1.6 A. The A36 steel work piece was clamped by alligator clips on the cathode.In the case of avoiding pollution, the anode material also uses A36 steel.
The schematic diagram of the experimental setup is shown in Figure 1.The experimental parameter settings are divided into four teams: A, B, C, and D, described as follows: Team A: The voltages are 50, 100 and 200 volts (V), respectively; the current is 1.6 amps (A); the electrolyte is trisodium phosphate aqueous solution, the volume is 0.3 liters (L), and the concentration is 0.127 (M); the treated time is 30 minutes (min); solution temperature is room temperature about 23 o C, such as Table 2 shows.Team B: The voltages are 50, 100 and 200 volts (V), respectively; the current is 1.6 amps (A); the electrolyte is a mixture of trisodium phosphate 0.3 (L) concentration 0.127 (M) and sodium percarbonate 0.3 (L) concentration 0.067 (M); the treated time is 30 minutes (min); solution temperature is room temperature about 23 o C , as shown in Table 3.
Team C: The voltages are 50, 100 and 200 volts (V), respectively; the current is 1.6 amps (A); the electrolyte is trisodium phosphate aqueous solution, the volume is 0.3 liters (L), and the concentration is 0.127 (M); the temperature of aqueous solution is about 50 o C; the treated time is 30 minutes (min), shown in Table 4.
Team D: The voltages are 50, 100 and 200 volts (V), respectively; the current is 1.6 amps (A); the electrolyte is a mixture of trisodium phosphate 0.3 (L) concentration 0.127 (M) and sodium percarbonate 0.3 (L) concentration 0.067 (M); the temperature of aqueous solution is about 50 o C; the treated time is 30 minutes (min), as shown in Table 5.

Measurement apparatus
In terms of surface morphology and microstructure observation was used by scanning electron microscope (Scanning Electron Microscope, SEM, HITACHI S3000N, Japan), and energy-dispersive X-ray spectroscopy (EDS) was used to analyze the chemical composition of the material surface.In the hardness analysis of the material surface, the hardness analysis is carried out using a Micro Vickers Hardness Tester (Mitutoyo HM-100).Detect the difference in the surface hardness of the test piece after micro-arc discharge plasma.

Results and discussion
In this study, the micro-arc discharge plasma combined with phosphating technology was used to A36 steel improve its surface hardness.The electrolytes were used trisodium phosphate (Na 3 PO 4 ), sodium percarbonate (Na 2 CO 3 ), mixture of trisodium phosphate (Na 3 PO 4 ) and sodium percarbonate (Na 2 CO 3 ) which with environmentally friendly properties.The micro-arc discharge plasma process is used for metals of aluminium, titanium, magnesium and their related alloys.The workpiece is placed on the anode, mainly to generate oxides on the surface that can resist corrosion.However, the oxide generated by iron at the anode is mainly composed of ferric oxide hydrate Fe 2 O 3 ．nH 2 O and iron hydroxide [FeO(OH), Fe(OH) 3 ], and the structure is not dense and cannot achieve the effect of corrosion resistance.Therefore, placing the workpiece on the cathode is expected to generate a phosphide film with better corrosion resistance on the steel surface.

Surface morphology observation
The surface morphology of all workpieces were observed via scanning electron microscope (SEM) and laser scanning confocal microscope.The surface chemical composition were analysed by energydispersive X-ray spectroscopy (EDS).Figure 2 shows the surface morphology and three dimensional (3D) surface topography of A36 steel before treatment, which roughness is about Sa = 2.442 μm.The phosphating treatment were using micro-arc discharge plasma, electrolyte of phosphate (Na 3 PO 4 ), with different volts and solution temperature as shown in Figure 3.The results shown there are no obvious difference in the surface morphology of all the specimens in these experimental condition.The electrolyte were be change to using the concentration 0.067M of phosphate (Na 3 PO 4 ) and concentration 0.127M of sodium percarbonate (Na 2 CO 3 ) mixing solution.There are particles and films formed on the surface of A36 steel as show in Figure 4. Figure 4(a)-Figure 4(c) show the particle size growth on the surface is larger than 50 V at 100 V, but the particle size seems to become smaller at 200 V again.When the temperature of mixing solution is adjusted to 50 o C, the surface formation seems to be less as show in Figure 4(d)-Figure 4(f).For nano or micro size of surface microstructure, two-dimensional (2D) images are difficult to observe, but three-dimensional (3D) images are easier.The three dimensional (3D) surface topography and roughness of A36 steel after phosphating treatment were observed and measured with laser scanning confocal microscope.Figure 5 and Figure 6 show the three dimensional (3D) surface topography of A36 steel after phosphating treatment.The results show as Table 6.In the experimental parameters of team A, single electrolyte solution of phosphate (Na 3 PO 4 ) and different volts, the surface roughness variation as 50 V, Sa = 2.673 μm; 100 V, Sa = 2.565 μm; 200 V, Sa = 2.539 μm, respectively.When electrolyte solution temperature increasing to 50 o C, the experimental parameters of team C, the surface roughness variation as 50 V, Sa = 2.540 μm; 100 V, Sa = 2.397 μm; 200 V, Sa = 2.475 μm, respectively.The variation of surface roughness in experimental parameters of team B were 50 V, Sa = 2.639 μm; 100 V, Sa = 2.806 μm; 200 V, Sa = 2.065 μm, respectively.The variation of surface roughness in experimental parameters of team D were 50 V, Sa = 2.374 μm; 100 V, Sa = 2.105 μm; 200 V, Sa = 2.691 μm, respectively.Figure 7 show the surface roughness variation trend of A36 steel with phosphating treatment.From the whole trend graph, the larger and smaller surface roughness values appear in the experimental parameter team B. The variation trend of surface roughness decreases with the increase of voltage, but increases again at 200V, in the experimental parameter team A and team C. The surface roughness variation value of team A is greater than that of team C. In the mixed electrolyte of experimental parameters B and D groups, the surface roughness has a large variation trend.The variation trend of surface roughness is increasing with the increase of voltage then decreasing in the experimental parameter team A. However, the variation trend of surface roughness decreases with the increase of voltage then increases in the experimental parameter team D.

Chemical composition of surface deposits analysis
From the results of surface morphology found thickness of phosphide deposits on A36 steel surface is too thin.Detect changes in surface chemical composition to determine if phosphide deposits were present.The chemical composition of A36 surface formation was detected using energy-dispersive Xray spectroscopy (EDS).Table 7 shows that the amount of phosphorus element about 0.49 Wt.% was detected in electrolyte solution of Na

Surface hardness variation
The surface hardness analysis was measured using a Micro Vickers Hardness Tester (Mitutoyo HM-100), the results as Table 8.The surface hardness before phosphating treatment was about 112. Figure 8 shows the surface hardness variation trend of A36 steel with the micro-arc discharge plasma phosphating treatment.All surface hardness after phosphating treatment is higher than that before untreated.Only in the conditions of 50V and 200V in team A, the hardness is lower than that before untreated.The variation trend of the surface hardness increases first and then decreases with the voltage in the whole experiment.However, the surface hardness decrease with the voltage increasing then increase in the team B. The maximum hardness value can be obtained when the voltage of team B is 100V.

Reaction mechanism
The influencing factors of micro-arc discharge plasma phosphating treatment can be divided into intrinsic and extrinsic factors.Intrinsic parameters are the composition and pH value of the electrolyte.The extrinsic factors consist of the type of voltage currently applied to the system, other inputs like the temperature of the electrolyte [19].The experimental parameters controlled in this study are the composition of the electrolyte and the temperature of the solution.The trisodium phosphate (Na 3 PO 4 ) and sodium percarbonate (Na 2 CO 3 ) were used as electrolyte components, which dissolve in water and was dissociated as follows [21]: (1) Sodium phosphate was dissociate in aqueous solution to form sodium cations, Na + and phosphate anions PO 4

Conclusion
In this study, the micro-arc discharge plasma combined with phosphating technology was used to A36 steel improve its surface hardness.The electrolytes were used trisodium phosphate (Na 3 PO 4 ), trisodium phosphate (Na 3 PO 4 ), mixture of trisodium phosphate (Na 3 PO 4 ) and sodium percarbonate (Na 2 CO 3 ), the results obtained make the conclusions as following: (1) The larger and smaller surface roughness values appear in the experimental parameter of mixture of trisodium phosphate (Na 3 PO 4 ) and sodium percarbonate (Na 2 CO 3 ) (team B). ( 2) The maximum hardness value can be obtained when the voltage of team B is 100V.
(3) The highest phosphorus content occurs in 100 V of team B. (4) The micro-arc discharge plasma combined with phosphating technology can used in steel surface to improve surface hardness.

Figure 7 .
Figure 7.The surface roughness variation trend of A36 steel with the micro-arc discharge plasma phosphating treatment.
3 PO 4 with 200 V.The amount of phosphorus element were detected about 0.15, 0.18 and 0.18 Wt.%, in electrolyte solution of Na 3 PO 4 and solution temperature about 50 o C, with 50 V, 100 V and 200 V, respectively.The amount variation of phosphorus element in Na 3 PO 4 and Na 2 CO 3 mixture solution were 1.38 and 0.54 Wt.% with 100 V and 200 V, respectively.The phosphorus element wasn't detected in 50 V of team B. The amount variation of phosphorus element in Na 3 PO 4 and Na 2 CO 3 mixture solution, solution temperature about 50 o C, were 0.44 and 0.36 Wt.% with 50 V and 100 V, respectively.The phosphorus element wasn't detected in 200 V of team D. The highest phosphorus content occurs in 100 V of team B.
29 HV.After phosphating treatment, the surface hardness values in team A are about 103.73 HV, 133.64 HV and 110.29 HV with 50 V, 100 V and 200 V, respectively.The surface hardness values in team C are about 145.6 HV, 113.09HV and 125.32 HV with 50 V, 100 V and 200 V, respectively.The surface hardness values in team B are about 144.29 HV, 154.40 HV and 140.50 HV with 50 V, 100 V and 200 V, respectively.The surface hardness values in team D are about 128.8 HV, 142.35 HV and 121.79 HV with 50 V, 100 V and 200 V, respectively.

Figure 8 .
Figure 8.The Surface hardness variation trend of A36 steel with the micro-arc discharge plasma phosphating treatment.

Table 1 .
The

Table 2 .
The experimental parameters of team A.

Table 3 .
The experimental parameters of team B (mixed electrolyte).

Table 4 .
The experimental parameters of team C.

Table 5 .
The experimental parameters of team D (mixed electrolyte).

Table 7 .
Chemical composition of surface deposits.