JPS61235573A - Chemical conversion treatment for steel sheet - Google Patents

Abstract

PURPOSE:To prevent generations of red rust and corrosion, by carrying out washing while using alkaline aq. soln. in washing process, at washing after formation of chemical conversion film on steel sheet surface. CONSTITUTION:Hanger 6 to which an automobile body 2 is fixed is transferred by a line conveyer 1, dipped into conversion liquid in a chemical conversion treating vessel 3, and conversion film is formed on steel sheet of the body 2. Next, alkaline aq. soln. is used to either one or both of washing water jetted to the body 2 taken out from the vessel 3 from a spray nozzle 4, and that of a washing vessel 5 for dipping the body 2 together with the hanger 6, and at last, water is jetted to perform washing. For alkaline eq. soln., that having <=11 pH is used. In this way, pH of liquid adhered to fine gap part and inner surface of bag shaped structure part of the body 2 is raised, generation and progress of red rust under low pH are suppressed, and corrosion in inner surface is prevented.

Description

[Detailed description of the invention] (Industrial application field) This invention relates to a method for chemical conversion treatment of steel plates.

(Prior art) Conventionally, the chemical conversion treatment method for steel sheets involves treating the steel sheet with a zinc phosphate-based or iron phosphate-based chemical conversion coating agent by spraying or dipping to form a chemical conversion coating on the surface. , a method of cleaning with at least one water washing step is used.

FIG. 1 is a diagram showing a chemical conversion treatment step before painting an automobile body as an example of a method for chemical conversion of steel plates. A chemical conversion film is formed on the steel plate of the vehicle body 2 by immersing it in a zinc phosphate-based chemical conversion coating agent. By spraying clean water onto the car body that has exited the chemical conversion treatment tank 8 using the spray nozzle number, further immersing it in the water in the washing tank 5, and after exiting the tank, by further jetting clean water, Cleans ions and impurities adhering to the surface of the steel plate (see New Automotive Engineering Handbook 911, Chapter 1 IL Painting, published by the Society of Automotive Engineers of Japan, May 1982).

(Problems to be Solved by the Invention) However, in such conventional chemical conversion treatment methods for steel plates, mildly acidic to neutral tap water is used as the washing water sprayed onto the car body. Also, since it takes a relatively long time from when the car body leaves the chemical conversion treatment tank 8 to when it goes to drying and electrocoating, it is necessary to remove minute gaps such as joints and narrow spaces in the treated steel plate or bag structure parts. In the case of automobile bodies, etc., it is not possible to neutralize the acidic chemical liquid that has entered the inner surface of such parts during the chemical conversion process, and the chemical coating coating does not spread well on those inner surfaces to begin with. There was a problem that corrosion of the anode was accelerated, which could lead to the occurrence of spring problems.

(Means for Solving the Problems) This invention treats a steel plate with a chemical conversion agent, which is a chemical conversion agent, to form a chemical conversion film on the surface of the steel plate, and then, relatively early in the process, the process of washing with water is performed. In both cases, the above problems are solved by washing with an alkaline aqueous solution in one step.

The alkaline aqueous solution used in this invention preferably has a pH of 11 or less in consideration of the stability of the chemical conversion film.

(Function) The method of the present invention can be advantageously used particularly when forming a steel plate having minute gaps such as joint openings and hem portions and bag structure portions, such as automobile bodies.

When an alkaline aqueous solution is injected or immersed into a steel plate treated material such as a car body that has been treated in a chemical conversion treatment tank using the method of this invention, the alkaline aqueous solution that has entered the minute gaps and the inner surface of the bag structure remains in the gaps. Neutralize the chemical conversion treatment solution,
It prevents the pH of the metal anode surface from decreasing, and the alkaline reagent such as sodium hydroxide in the solution helps passivate dissolved oxygen, thereby suppressing rusting under acidic conditions.

Furthermore, when sodium nitrite is added to adjust the pH, this sodium nitrite also has the effect of passivating the metal as a corrosion inhibitor.

(Embodiment) In the present invention, as in the example shown in FIG. 1, the hanger 6 to which the vehicle body 2 is fixed by the line conveyor 1 is moved in the same manner as in the conventional method, and the hanger 6 is immersed in the chemical solution in the chemical conversion treatment fRδ. A chemical conversion film is generated on the steel plate of the vehicle body 2. Next, in this invention, an alkaline aqueous solution is used for either or both of the washing water injected from the spray nozzle 4 onto the car body 2 that has exited the chemical conversion treatment tank 8, and the washing water from the washing tank 5 in which the hanger 6 is immersed. Finally, spray water to clean.

Next, a concrete explanation will be given with reference to Examples and Comparative Examples.As shown in FIG. After that, two pieces of Q and 35 m polyester spacer 12 were placed facing each other and used as test pieces.

The pre-processes for chemical conversion treatment include hot water washing (spray) and degreasing (F
O4157, manufactured by Nippon Parr Ising Co., Ltd., dip), water washing (spray), and surface conditioning (PL4081, Nippon Parr Ising Co., Ltd., dip). Thereafter, a chemical conversion treatment (BT 8004, manufactured by Nippon Bar Ising Co., Ltd., DIP) was applied using the chemical conversion treatment equipment shown in Fig. 2, and after taking out the test piece from this chemical treatment tank for 10 seconds, approximately 1
After 0 seconds, water washing after chemical formation was performed by spraying with a tin sprayer for 10 seconds, followed by dipping water washing for 60 seconds. In FIG. 2, 7 is a chemical conversion experimental tank, 8 is a liquid circulation pump, 9 is a liquid circulation hose, 10 is a surface flow layer nozzle X11 is an electric heater. In the chemical conversion treatment, each test piece is immersed in chemical liquid BT8004 in a chemical treatment experimental tank 7, the temperature of the liquid is maintained at 50°C with an electric heater 11, and the chemical liquid is pumped from the bottom of the tank through a liquid circulation hose 9 with a pump 8. Nozzle 1 for surface flow layer through
The sample was immersed for 2 minutes while circulating the liquid under pressure.

The aqueous solution used in the spraying and dipping water washing after chemical conversion was prepared in the following manner.

First, for one test piece, 8
2.71N of 5% phosphoric acid, 2.86N of 96+% glacial acetic acid, and 2.4N of boric acid (H, BO8) were dissolved in ion-exchanged water to make a total volume of 1000N. To this acid mixture, 775 ml of 0.2N sodium hydroxide was added, and then a small amount of sodium nitrite was added while checking the pH using a pH meter to adjust the pH to 9.8.

Further, as a comparative example, an experiment was conducted using tap water with a pH of 6.8.

As a result of the above experiment, red rust occurred over a wide area on the test piece of the comparative example, whereas on the test piece of the experimental example, red rust was only slightly formed locally, and the neutralization of the chemical liquid and alkaline aqueous solution The reaction produces an insoluble salt (F8PO, and HOpite Zn).

(PO, 8.4H80 is considered) is produced, but since these are produced in small amounts and can be washed away at the same time as they are produced in the water washing process by spraying and immersion, it can be judged that the present invention does not have any negative effects.

(Effects of the Invention) As described above, according to the present invention, in the chemical conversion treatment of steel sheets, cleaning water after the chemical conversion treatment is used in an alkaline aqueous solution, preferably an alkaline substance, an acidic substance, and a corrosion inhibitor. Since we use an aqueous solution that has been adjusted to be alkaline with one or more substances from This has the effect of increasing the pH of the solution, inhibiting the occurrence and progression of red rust under low pH conditions, and also inhibiting corrosion on the inner surface due to the passivation effect of the inhibitor.

[Brief explanation of the drawing]

FIG. 1 is a process diagram of a chemical conversion treatment for an automobile, FIG. 2 is a sectional view of a chemical conversion treatment apparatus used in Examples and Comparative Examples, and FIG. 8 is a perspective view of a test piece. 1...Line conveyor 2...Car body 3...Chemical treatment tank 4...Spray nozzle 5...Washing tank
6...Hanger 7...Chemical treatment experimental tank 8.
...Liquid circulation pipe 9...Liquid circulation hose 10...
Surface flow layer nozzle 11...Electric heater 12...
・Polyester spacer 18...Steel plate Figure 1 Figure 2 Hole 9

Claims (1)

[Scope of Claims] 1. A method for chemical conversion treatment of a steel plate in which a steel plate is treated with a chemical conversion coating agent to form a chemical coating on the surface of the steel plate, and then washed with water, at least one step of the washing process after the formation of the chemical coating. 1. A method for chemical conversion treatment of steel sheets, which comprises cleaning with an alkaline aqueous solution.