JPH0411630B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming a phosphate conversion coating, such as zinc phosphate, on a steel surface.

Phosphate conversion coatings are used as a coating base for steel plates for the purpose of preventing rust and improving adhesion, and for improving lubricity.
It is used by forming it on the surface of steel materials for friction sliding. In the conventional formation of phosphate conversion coatings, the temperature of the treatment bath is set at 40℃ or higher, and the total acid, free acid, oxidizing agent, etc. in the treatment bath are determined by chemical capacity analysis, and the results are
Taking into account the judgment of the operator's experience, phosphate ions,
The main material containing metal ions such as zinc and the auxiliary agent containing nitrite ions were replenished in determined amounts, the treatment bath was controlled, and a phosphate film was formed. However, chemical capacitance analysis takes a long time to produce results, and changes that appear to be abnormal reactions occur in the treatment bath, making it difficult to adequately manage the bath even when the experience of the operator is taken into account. As a result, the quality of the phosphate conversion coating produced varies widely, and when a steel plate is coated, problems such as easy rusting occur.

By the way, processing baths that are used at low temperatures of 30°C or less are also known, but when such processing baths are used in continuous processing, the components of the processing bath change sequentially, so Control is required.
However, as described above, conventional methods have relied on chemical capacitance analysis to control the components of the processing bath, resulting in similar problems.

The inventor studied the above-mentioned problem from the viewpoint of the chemical reaction of the processing bath, and discovered that when the processing bath is used at high temperature, non-electrochemical reactions are strong and abnormal reactions are likely to occur. It was discovered that when used at low temperatures, the electrochemical general corrosion reaction is predominant, the reaction is stable, the treatment bath can be easily managed, and a dense phosphate conversion coating can be obtained.

That is, the method of forming a phosphate chemical conversion film on the surface of a steel material according to the present invention requires the temperature of the treatment bath to be 0°C or higher.
40℃ or less, and the hydrogen ion concentration and redox potential of the treatment bath are PH0.5 to PH5.0 and PH0.5 to PH5.0, respectively.
300mV to 700mV (Hydrogen standard electrode potential, same below)
This feature is characterized in that processing is performed while maintaining the range of .

The basic components of the processing bath used here are almost the same as those of conventional processing baths. In other words, the components of the treatment bath are a main agent containing metal ions such as phosphate ions, nitrate ions, and zinc, and an auxiliary agent containing an oxidizing agent such as nitrite ions. It is dissolved.

The metal ions contained in the main ingredient are not limited to zinc, but iron, manganese, calcium, magnesium, etc. Similar to zinc, they exist as hydrogen phosphate compounds that are stable in aqueous solutions, and the dehydrogenation shown in the following formula (1) Those with decreased solubility can be used.

xM(H ₂ PO ₄ )y→Mx(PO ₄ )y +2yH ⁺ (1) Other metal ions other than zinc, such as nickel and titanium, which are generally included in the main ingredient as other components, are (1) It is added to efficiently perform the dehydrogenation oxidation reaction of the formula, and can be used in the processing bath of the present invention as well as in conventional processing baths.

Nitric acid and chloric acid, which are acid components other than phosphoric acid contained in the base agent, play a role in dissolving film-forming metal components such as zinc in the treatment bath.

In the case of the present invention, the mixing ratio of phosphoric acid and metal ions is 100 parts by weight of phosphoric acid to 100 parts by weight of metal ions such as zinc.
It is preferably 60 to 100 parts by weight, more preferably 70 to 95 parts by weight. This ratio has a large amount of metal ions compared to conventional high temperature baths. This is because in the present invention, since the treatment is performed at a low temperature, there is less dissolution of the material to be treated and less decomposition of phosphoric acid.

Nitric acid and chloric acid correspond to the electrochemical anode deposition reaction in which film-forming metal components such as zinc are shown in equation (1), and electrochemical cathode precipitation reactions shown in equations (2), (3), and (4). Decomposes by decomposition reaction. In addition,
The potentials (V) shown in (2), (3), (4) and equation (10) described below indicate the hydrogen standard electrode potential of each reaction formula at 25°C.

NO ₃ ^- +2H ⁺ +2e→NO ₂ ^- +H ₂ O 0.94V (2) NO ₂ ^- +2H ⁺ +e→NO↑+H ₂ O 1.0V (3) CIO ₃ ^- +2H ⁺ +2e→CIO ₂ ^- +H ₂ O 1.21V (4) As the oxidizing agent contained in the auxiliary agent, those that undergo an oxidation reaction accompanied by hydrogen ions, as shown in (3), can be used. Sodium nitrite, which has been used conventionally, is suitable.

Note that injecting nitrite ions, etc. into the processing bath as an auxiliary agent is to decompose only the cathodic reaction seen in equations (2) and (4) of nitrate ions or chlorate ions that have been added to the processing bath in advance. So,
(1) The total anode reaction in the treatment bath ((10),
(11), (12), etc.) cannot be accommodated in terms of reaction rate. Therefore, from a thermodynamic point of view, a chemical that is likely to react (has a high reaction rate) (for example, sodium nitrite, etc.) is separately added to the treatment bath as an auxiliary agent.

In the method of the present invention, the temperature of the treatment bath is 0 to 40℃.
This is because the non-electrochemical reactions that occur in the treatment bath in conventional methods are suppressed and the conversion coating is generated based on an electrochemical general corrosion reaction. If the treatment bath is used at a high temperature as in conventional methods, thermal decomposition tends to proceed. Generally, at high temperatures, the entropy (△S) of a chemical reaction system is
As a result, non-electrochemical reactions such as thermal decomposition reactions become stronger. In the phosphate treatment bath, in addition to the reactions of equations (3), (4), and (5) described above, it is thought that the following thermal decomposition reactions of equations (6) and (7) become stronger.

NO ₂ ^- →NO ₂ ↑+e (6) H ₃ PO ₄ →H ⁺ +H ₂ PO ₄ ^- (7) As a result of the reactions in equations (6) and (7), the reactions shown in equations (8) and (9) It is thought that this will progress.

H ⁺ +e→1/2H ₂ ↑ (8) 3Zn ²⁺ +2H ₂ PO ₄ ^- +4e→ Zn ₃ (PO ₄ ) ₂ ↓+2H ₂ ↑ (9) Therefore, in a high temperature treatment bath, equation (6) Nitrite ions are consumed by the reaction and NO ₂ gas is generated, and H ₂ gas is generated by the reaction of equation (8).
Then, by the reaction of equation (9), sludge ``Zn ₃ (PO ₄ ) ₂ }
occurs. For this reason, in a high-temperature treatment bath, the components of the treatment bath are consumed as NO ₂ gas, H ₂ gas, and sludge, and it is now necessary to add more components to the treatment bath than those required to form a phosphate film.

In the method of the present invention, since the temperature of the treatment bath is kept at 40° C. or lower, the reactions of formulas (6) and (7) above can be greatly suppressed. Therefore, cations and anions in the treatment bath can exist stably, and (8)
The reaction of equation (9) is also suppressed, and the generation of H ₂ gas and sludge is reduced.

In the method of the present invention, a full-scale electrochemical corrosion reaction occurs on the steel surface, which is thought to result in a phosphate film formation reaction. Here, the full surface electrochemical corrosion reaction refers to a reaction in which an anode reaction (oxidation reaction such as metal dissolution) and a cathode reaction (reduction reaction) occur at the same location. In this reaction, corrosion (dissolution) of the steel occurs uniformly, and by appropriately selecting conditions such as anion composition and concentration, a film of corrosion products is uniformly formed on the steel surface. Subsequent melting of steel is suppressed. This coating consists of chromium oxide (CrO ₃ ) on the surface of austenitic stainless steel.
It can be considered to be similar to the formation of a film.

The anode reaction in this general electrochemical corrosion reaction is the reaction of equations (10), (11), and (12) Fe→Fe ²⁺ +2e−0.44V (10) Fe ²⁺ +H ₂ PO ₄ ^- →FePO ₄ ↓+2H ⁺ +e (11) 3Zn ²⁺ +2H ₂ PO ₄ ^- →Zn ₃ (PO ₄ ) ₂ ↓＋4H ⁺
(12), and the cathode reactions are reactions shown in equations (2), (3), and (4). These reactions cause phosphate (FePO ₄ ), zinc phosphate, and {Zn ₃ (PO ₄ ) ₂ } to precipitate on the steel surface.

In order for these phosphate film forming reactions to be adopted in general production lines, the reaction rate must be sufficiently fast. In a chemical reaction at an electrode, the factors that influence the reaction rate are the concentration of substances involved in the reaction, temperature, pressure, and electrode potential. Here, the higher the temperature, the faster the reaction rate, but (6), (8), (9)
In order to prevent the interfering reactions associated with the gas generation shown in the equation, it is necessary to lower the temperature. In the case of the immersion method, the pressure is usually constant at atmospheric pressure, but in the case of the spray method, the higher the pressure, the better. Regarding the concentration of reactants, in the dissolution reaction of equation (10), NO ₂ ^-
The higher the concentration of oxidizing agents such as hydrogen ions and hydrogen ions, the better.
In the film formation reactions of equations (11) and (12), the concentration of hydrogen ions must be below a certain level. Regarding the electrode potential, it is necessary that at least the reaction potential of the oxidizing agent (cathode reaction potential) is larger (higher) than the dissolution reaction potential (anode potential) of the steel.

From the above, in order for the reaction to form a phosphate film on the steel surface to proceed at a constant rate as an electrochemical reaction between 0°C and 40°C, (a) materials and treatments that dissolve sufficiently quickly at room temperature are required. (b) At room temperature, it is necessary to maintain the concentration of the oxidizing agent and hydrogen ion concentration of substances involved in the reaction in a range that allows the formation of a stable phosphate film.

When the material to be treated is steel, a treatment bath made from a combination of a conventional main agent consisting of phosphoric acid, nitric acid, zinc, etc., and an auxiliary agent mainly consisting of a conventional nitrite as an oxidizing agent satisfies the condition (a). do. Regarding the concentration of reaction-participating substances in the treatment bath, it is necessary that there is a sufficiently small amount of sludge in the treatment bath. Under these conditions, the pH ranges from PH0.5 to PH5.0, and the concentration of the auxiliary agent is oxidation-reduction. Satisfies condition (b) with a potential (OPR value) of 300 to 700 mV.

In addition, in conventional treatment baths used at high temperatures, the pH is generally in the range of PH3.0 to PH3.4 in the case of spray type treatment baths. In case of immersion type treatment bath, PH2.0~
In the range of 2.5. In the method of the present invention, since the treatment bath temperature is set to 40° C. or lower, sludge is difficult to form in the bath, and as a result, the reactions of equations (11) and (12) occur on the steel surface. Therefore, it is possible to widen the PH value of the treatment bath according to the present invention to a range of PH0.5 to 5.0. In addition, when the pH is lower than 0.5, the reactions of equations (11) and (12) are difficult to proceed, and the film formation reaction is suppressed.

In the case of phosphate treatment baths, the measurement of PH and ORP values is as follows:
When lowering the temperature from a high temperature to a low temperature, the equilibrium reaction in the treatment bath changes, for example, as traditionally shown by an increase in the "free acid concentration."
Both PH and ORP values are expressed differently at high and low temperatures. The PH and ORP values referred to in this specification are values measured at the operating temperature of the treatment bath.

The oxidation-reduction potential of the treatment bath related to the method of the present invention is
It is in the range of 300-700mV (hydrogen standard electrode potential).
This is lower than the redox potential of conventional treatment baths used at high temperatures, which is 730 mV or more. This is because in conventional processing baths, as shown in equations (6) to (9), heating accelerates the self-decomposition reaction of bath components. It is thought that it exhibits a high redox potential due to the synergistic effect of requiring an oxidizing agent and high temperature heating. From another perspective, in high-temperature baths, there is a large amount of sludge of zinc phosphate, which is the same component as the coating, so a large amount of force is required to promote the coating formation reaction on the steel surface, and therefore heating is required. shall be. A large amount of the oxidizing agent, which is the other substance involved in the reaction, is also used, resulting in a high redox potential, and unless the redox potential is kept high at all times, it is impossible to form a film.

In the treatment bath of the method of the present invention, since only a small amount of sludge is present in the bath and the temperature is low, the reaction can proceed electrochemically and ideally without waste, compared to conventional baths. Areas with a wide pH range and low redox potential (700mV or less)
It is considered that sufficient film formation reaction can be carried out with the following steps.

FIG. 1 shows the PH and redox potential ranges of the conventional treatment bath and the treatment bath used in the present invention. The rectangular range indicated by the symbol A in FIG. 1 is the range of PH and redox potential according to the present invention. Also, the range indicated by the symbol P is the treatment bath by the conventional method.
range of pH and redox potential.

The metal material to be treated that can be treated by the method of the present invention is steel. Here, the term "iron and steel" includes not only ordinary iron and steel but also alloy steel such as stainless steel, and surface-treated steel such as galvanized steel sheet.

Management of the treatment bath according to the present invention can be automated by measuring the PH and redox potential of the treatment bath because the film forming reaction is electrochemically carried out. When steel is processed, the main and auxiliary components are removed from the treatment bath. The concentrations of the main component and the auxiliary component in the treatment bath are correlated with the PH value and redox potential. That is, when the main agent component decreases, the PH of the treatment bath increases, and when the auxiliary agent component decreases, the oxidation-reduction potential of the treatment bath decreases. For example, when replenishing the main ingredient, you can replenish a fixed amount of the main ingredient when the pH rises above 3.0, or open the main ingredient replenishment valve when the pH rises above 3.0, and replenish the main ingredient when the pH drops below 2.5. The valve may be closed. Alternatively, a method may be used in which the main agent is constantly reinforced, and when the pH rises above a certain value, the amount of supplementation is increased, and when the pH falls below a certain value, the amount of supplementation is decreased.

In addition, PH control is also possible by separately injecting the components of the base agent, such as zinc, zinc oxide, phosphoric acid, nitric acid, etc., instead of combining them as a base agent. Sometimes caustic soda can also be used. In that case, zinc oxide and caustic soda are injected to increase the PH value (reducing the hydrogen ion concentration), and nitric acid and phosphoric acid are injected to lower the PH value.

The same applies to the replenishment of the auxiliary component, for example, a method may be used in which the auxiliary replenishment valve is opened when the redox potential becomes 400 mV or less, and the valve is closed when the oxidation-reduction potential becomes 500 mV or more. Both PH value and redox potential are measured electrically, and chemical analysis is not required, making it very simple. Therefore, the above-described management method can be easily automated. The main components of the treatment bath include, for example, A [zinc 15,000 to 18,000 ppm, phosphate ion 15,000 to 20,000 ppm, nitrate ion 9,000 to
15000ppm, chlorate ion (CIO ₃ ^- ) 1000~
2000ppm, nickel 40-60ppm]
Another example is B [zinc 15,000 to 20,000 ppm,
Phosphate ion 15000~20000ppm, nitrate ion 7000~
14,000 ppm, chelate 200-400 ppm] can be used. The replenishing solution for the main ingredient is a 10 to 20 times concentrated solution of the above components, which can be used by replenishing the necessary amount into the bath. Further, as an auxiliary agent, an aqueous solution containing about 20 to 50% by weight of sodium nitrite (NaNO ₂ ) can be used, and it is added to the baths A and B for use. It is contemplated that other oxidizing agents such as sodium chlorate may also be used. For reference, the content (points) and redox potential (mV) of sodium nitrite in the treatment bath based on conventional chemical analysis.
The relationship is shown in Figure 2. The solid line in FIG. 2 is a diagram showing the relationship between the auxiliary agent concentration and the redox potential when the treatment bath temperature is 25 DEG C. to 30 DEG C., the pH is 2.9, and the amount of sludge in the bath is sufficiently small. From FIG. 2, it can be seen that when the temperature of the treatment bath is low and there is little sludge, there is a certain correlation between the auxiliary agent concentration and the redox potential. Incidentally, the relationship between the concentration of the auxiliary agent and the redox potential changes depending on the type of auxiliary agent and the type of the main agent used.

The phosphate conversion coating obtained by the treatment method of the present invention is denser than the coating obtained by conventional methods. For this reason, the corrosion resistance of the paint film and the elongation of the film during cold forging press processing are excellent. The reason why such a superior film can be obtained can be explained from empirical rules regarding electrochemical reactions on metal surfaces such as plating processing. Empirically, it is known that when the anions in the solution have the same composition and concentration, the higher the overvoltage on the metal (electrode) surface, the more dense the deposit (film) can be obtained and the more stable the film. ing. On the other hand, it is known that the overvoltage on a metal surface decreases rapidly as the temperature rises, and that the higher the temperature, the more unstable a film with coarse crystals can be obtained. From these facts, since the temperature of the treatment bath according to the method of the present invention is lower than that of conventional treatment baths, the film produced by the method of the present invention is produced under a high overvoltage, and therefore the resulting film is not dense. It is considered to be stable.

Furthermore, compared to conventional methods, the method of the present invention not only provides a dense and stable phosphate coating, but also
Since the treatment bath can be managed by measuring the PH value and redox potential, it is easier to manage the treatment bath than in the past, and automatic management is also easier. Furthermore, since the temperature of the processing bath is 0 to 40°C, which is normal temperature, there is no need to heat the processing bath as in the conventional method. Therefore, energy consumption can be reduced. Furthermore, since there is little self-decomposition reaction of the processing agent, the processing agent can be used efficiently, reducing the amount of processing agent used to 1/3 compared to conventional processing baths.
It can be reduced to: This is based on the fact that sludge formation can be significantly reduced. Furthermore, a settling tank, which was conventionally required for a processing bath, is no longer necessary, and the equipment is simplified.

Examples will be explained below.

As shown schematically in Figure 3, zinc ions
15000~18000ppm, phosphate ion 15000~
A main agent supply pipe 22 is supplied from a main agent tank 2 via a solenoid valve 21 to a processing tank 1 holding a 0.7 ^m3 treatment bath containing 20,000 ppm of nitrate ions, 9,000 to 15,000 ppm of nitrate ions, 1,000 to 2,000 ppm of chlorate ions, and 40 to 60 ppm of nickel. An auxiliary agent supply pipe 32 was connected from the auxiliary agent tank 3 via a solenoid valve 31. Then, these solenoid valves 21 and 31 are connected by an electric circuit (not shown) that is opened and closed by a PH meter 23 and an oxidation-reduction potentiometer 33 immersed in the processing bath.
When the pH reaches 3.5 or more, the valve 21 opens and the main agent is supplied from the main agent tank 2 into the processing tank 1 until the pH reaches 2.5.
On the other hand, the valve 21 is closed when the
When the redox potential meter (silver chloride electrode) 33 becomes 400 mV or less (based on hydrogen standard electrode potential), the solenoid valve 31 is opened, the auxiliary agent is supplied from the auxiliary agent tank 3 into the processing tank 1, and the oxidation-reduction potentiometer 33 is 350mV
When the temperature exceeds that level, the solenoid valve 31 closes. A spray pipe 4 is provided on the side wall of the processing tank 1, and a processing bath is sprayed onto the surface of the material W to be processed from a spray nozzle row 6 provided above the processing tank 1 in two stages, upper and lower, via a pump 5. I made it.
The main ingredients for replenishment are 25g of zinc, 27g of phosphoric acid, 20g of nitric acid, and 3g of chloric acid per minute. An aqueous solution containing 3 g of nickel was supplied per minute, and an aqueous solution containing 40 g of sodium nitrite per minute was also supplied as an aid for replenishment. In addition, a cup-shaped cover for an automobile starter press-formed from a cold-rolled steel plate and having a diameter of approximately 9 cm was used as the material to be treated. This material to be treated is washed with 55℃ hot water for 0.5 minutes → sprayed with 55℃ alkaline aqueous solution for 2 minutes to degrease → washed with 45℃ hot water for 0.5 minutes → room temperature (20 to 30℃)
Spray cleaning for 0.5 minutes with water → Phosphate conversion coating treatment by spraying a treatment bath at room temperature (20 to 30℃) for 2 minutes using the equipment shown in Figure 3 → Spray cleaning for 0.5 minutes with room temperature water → 0.5 minutes with room temperature water Spray cleaning for 2 minutes followed by drying with hot air at 80 to 90°C for 2 minutes to form a phosphate chemical conversion film mainly containing iron phosphate and zinc phosphate on the surface of the treated material.
In addition, 1,500 pieces were processed in one hour using this device, and all processing baths were managed automatically. in this state
The treatment was carried out for 180 days, and no abnormality was observed in the treatment bath during that time.

For reference, a record of automatic control of the processing bath is shown in FIG. The PH adjustment system uses a UHC-76-6045 PH electrode manufactured by Denki Kagaku Keiki Co., Ltd.
An HBR-92 type control recorder was used. Figure 4 shows a part of the PH recorder. In Figure 4, the horizontal axis shows the PH value and the vertical axis shows time. One section on the vertical axis corresponds to one hour. In the range shown by A in Figure 4, replenishment of the main agent is started when the pH is 3.0, and the pH drops to 2.5 in about 3 hours.
Indicates a state in which main agent supply has been stopped. During this time, the phosphoric acid film formation treatment in the bath was also carried out. In the range shown in Figure 4, hydrogen ions are consumed as the treatment bath is used without replenishment of the base agent, and the pH
shows a gradual increase.

The reason why the pH value in the bath fluctuates slowly is because the dissociation constant of phosphoric acid is small, so slight fluctuations in the base agent
This is because it does not lead to large fluctuations in the PH value.

FIG. 5 shows a part of the ORP value recorder. The horizontal axis shows the redox potential and the vertical axis shows time. One section on the vertical axis is one hour. This ORP adjustment system is UHC-76 manufactured by Denki Kagaku Keiki Co., Ltd.
-6026 type metal electrode (silver chloride electrode) and HBR-
A type 94 control recorder was used. Silver chloride electrodes are commonly used, and conversion to hydrogen standard electrode potential is performed using equation (13).

E (NHE) = E (AgCI) + 206 -0.7 (t-2.5) mV (13) E (NHE)...Hydrogen standard electrode potential E (AgCI)...3.33MoKcI-AgCI electrode potential t...Temperature (℃) Note that in displaying the PH and ORP values according to the present invention, as described above, the values are at the operating temperature, and the temperature coefficient of equation (13) is not taken into account.

The state shown in FIG. 5C is the state when the device starts operating. At this time, the workpiece (steel) has not yet been put into the processing bath. Therefore, ORP in the bath
The value is dominated by reaction potentials expressed by equations (2), (3), and (4), and is in a high potential state. Electrochemically, it can be said that the circuit is broken in the cathode reaction state.

Condition D is the state when the workpiece is put into the processing bath, and the anodic reactions (film formation reactions) of equations (10), (11), and (12) occur in correspondence with the cathodic reactions described above. The potential of the treatment bath drops rapidly.

In the case of E, the injection of the auxiliary agent was automatically controlled according to the ORP value, and the injection of the auxiliary agent was started when the ORP value decreased to 300 mV, and stopped when the ORP value reached 350 mV. be. As a result, the bath potential (ORP) is controlled within a certain range of 280 to 350 mV.

In the condition F, the potential has increased because the input of the workpiece (steel) has been temporarily cut off. The state immediately returns to E as soon as the workpiece is input.

The condition in (g) is the same as in (a), where there is no workpiece in the bath, and the input of the workpiece has been stopped.
The bath becomes insulated from the cathode reaction potential,
The ORP value increased rapidly.

As described above, in the method of the present invention, all treatment baths can be controlled automatically and electrochemically. Note that it is necessary to prevent an electrochemical reaction between the processing bath and the material of the tank, and it is preferable that the material of the processing tank is highly insulating (for example, using a rubber lining material).

The treated material on which the phosphate conversion coating was formed in this example was then spray-painted with a black urethane-epoxy resin paint, set for 3 minutes, and then baked for 6 minutes in a baking oven at 140°C. 12
A coating film thickness of ~18μ was obtained. After 48 hours had elapsed after baking, the coated product was subjected to a salt spray test according to JISK-5400-7.8 to examine the corrosion resistance of the coating film.
The results are shown in FIG. Reference numeral A in FIG. 6 is a graph showing the relationship between the salt water spray time and the rusted area of the coated product treated by the method of this example. Reference numeral B is a diagram of a painted object treated in a conventional manner. The zinc phosphate coating treatment of this example was performed using a conventional high-temperature bath of 40°C or higher (temperature 50-55°C, pH 3.1-3.3, redox potential
A significant improvement in corrosion resistance was observed compared to that treated with 730 to 750 mV (with the same base and auxiliary ingredients).

[Brief explanation of drawings]

Figure 1 is a diagram showing the range of PH and redox potential of the treatment bath according to the present invention and the conventional treatment bath, and Figure 2 is a diagram showing the range of auxiliary agent concentration and redox potential in the treatment bath according to the present invention. A diagram showing the relationship, FIG. 3 is a schematic diagram of the processing device used in the embodiment of the present invention, and FIG. 4 is a recording diagram of the PH value when performing the automatic PH control of the present embodiment.
Figure 5 is a recording diagram of ORP values when the ORP automatic control of this example was performed, and Figure 6 is a graph showing the salt spray time and rusting of painted products treated by the method of this example and the conventional method. FIG. 3 is a diagram showing the relationship between areas. In the figure, 1 is a treatment tank, 2 is a main agent tank, 3 is an auxiliary agent tank, 4 is a spray pipe, 5 is a pump, 6
indicates a spray nozzle row.

Claims (1)

[Scope of Claims] 1. A method of bringing a steel material into contact with a phosphate chemical conversion treatment bath containing a phosphate to continuously form a phosphate conversion coating on the surface of the steel material, comprising: controlling the temperature of the treatment bath to 0° C. or higher; The temperature is 40°C or lower, and the hydrogen ion concentration of the treatment bath is controlled within the range of PH0.5 to PH5.0, and the redox potential of the treatment bath is controlled within the range of 300mV to 700mV (hydrogen standard electrode potential). A method for forming a phosphate conversion coating on a steel surface. 2 The amount of zinc metal ions in the treatment bath is 100% phosphoric acid.
The method according to claim 1, wherein the amount is 60 to 100 parts by weight. 3 When the PH of the processing bath reaches a certain value or higher, the main agent containing metal ions such as phosphate ions, nitrate ions, and zinc is replenished into the processing bath, or the amount of replenishment is increased to bring the PH of the processing bath to 0.5 to 5.0. and when the oxidation-reduction potential of the processing bath falls below a certain value, the processing bath is replenished with an auxiliary agent containing an oxidizing agent such as nitrite ions, or the amount of replenishment is increased to reduce the oxidation-reduction potential of the processing bath. The method according to claim 1, wherein the potential is maintained in the range of 300 mV to 700 mV.