JPS62202081A - Formation of phosphate film on hot dip galvanized body - Google Patents

Abstract

PURPOSE:To easily form a zinc phosphate film on a hot dip galvanized body at room temp. and to remarkably improve the adhesion to a paint film by treating the hot dip galvanized body with a principal soln. contg. zinc phosphate and an auxiliary soln. of a prescribed pH. CONSTITUTION:An aqueous soln. contg. 10-30g/l Zn(H2PO4)2.2H2O and <=3g/l NaNO2 is prepd. as a principal soln. An aqueous soln. of <=2 pH contg. 50-65g/l sulfuric acid radicals and 65-85g/l NaOH is prepd. as an auxiliary soln. A hot dip galvanized body contg. Al in the zinc layer is treated with the principal soln. mixed with the auxiliary soln.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for forming a phosphate film on the surface of a hot-dip galvanized product and an aluminum-containing hot-dip galvanized product. More specifically, the invention particularly relates to thermosetting or thermoplastic resins (epoxy, polyester, acrylic, polyethylene, vinyl chloride, nylon resin, etc.) used in electrostatic coating, powder electrostatic coating, fluidized dipping, etc. This invention relates to a method for chemically forming a zinc phosphate film as a coating base on the surface of a hot-dip galvanized object so that its adhesion will not be lost even when exposed to high temperatures during treatment.

[Prior art] In general, chromate coating is used as a base for painting metals.
Chemical conversion coatings such as V-acid coatings are well known, and various chromates, manganese phosphate coatings, and zinc pyrophosphate coatings are used as bases for painting zinc and its alloy surfaces, providing good corrosion resistance and Adhesion to the paint film has been achieved.

However, these treatments usually require temperatures of 85°C or higher, and the processing capacity is large, ranging from several tens to 100 TIf.
As the immersion time gradually increases, it tends to become difficult to obtain a good film. Furthermore, the composition of the liquid is complex, and it is necessary to frequently analyze the composition over time, and liquid management such as addition of replenisher, water replenishment, and maintenance of liquid temperature is also troublesome.

Furthermore, an improved type of phosphate film forming treatment solution for hot-dip galvanizing containing aluminum has been provided in which 1-2 types of agglomerates are added as an aluminum treatment agent. Furthermore, there are improved treatment solutions that add a large amount of nickel to lower the treatment temperature (70°C or less) and make the film finer, but their treatment capacity is 1.
rd/jl, which is extremely small, and the ratio of free acidity to total acidity, which is necessary for the management of the treatment liquid, is also highly variable and cannot be said to be practical.

[Problems to be Solved by the Invention] Generally, the surface of hot-dip galvanized products commonly used for plating iron-based structures is covered with zinc oxide films of various thicknesses, and the underlying metal They range from those that are shiny to those that appear colored due to light interference. In addition, in some cases, molten lead (dross) containing several percent of iron as a solid solution, adherents such as flux thermal decomposition residue (kasuki) and floating slag, and zinc-iron alloy layers are exposed. Some of them are matte and gray in color, and phosphate treatment is preferable to electrogalvanizing.

However, there is a tendency to form an insufficient film that leaves these defects.

In addition, there is a method in which aluminum is added to the hot-dip galvanizing bath to improve the gloss and smoothness of the surface of the plated object, reduce the oxide film, suppress the growth of the zinc-iron alloy layer, and improve workability. Recently, hot-dip galvanized products (A, & containing 2.01 to 3.0% of N) have come on the market, with considerably different concentrations of aluminum contained in the bath. The surface of the plated object containing aluminum is 0.
At concentrations below 0'1% A, it is preferentially oxidized,
Covered with an extremely thin and stable aluminum oxide film, it has a strong metallic luster. If the concentration is higher than that, aluminum dissolved in molten zinc preferentially reacts with the base to form a zinc-iron-aluminum ternary alloy layer, thereby suppressing the development of the zinc-iron alloy layer. At the same time, aluminum contained in the plating film is preferentially oxidized during cooling to form a thin aluminum oxide film, and aluminum tends to concentrate very close to the surface. Even if such plated objects are subjected to phosphate treatment, depending on the concentration of aluminum, an incomplete phosphate film tends to be formed due to the influence of aluminum and its oxides, and it is difficult to use as a base for painting. Adhesive properties are often poor, making it unsuitable.

Therefore, the present invention is a method for forming a surface film belonging to phosphate treatment, which is more effective than the phosphate film formed by conventional methods on the surface of hot-dip galvanized and hot-dip galvanized objects containing aluminum.
An object of the present invention is to provide a method for forming a zinc phosphate film that exhibits excellent adhesion to paint films.

[Means for solving the problems] The present invention consists of a main liquid consisting of trizinc phosphate, phosphoric acid or zinc phosphate and nitrite, a sulfuric acid radical and sodium hydroxide, and the pH is adjusted to below 2. The auxiliary solution was added to the aluminum I! in the hot dip galvanized product. An aqueous solution mixed according to the temperature is brought into contact with the surface of the hot-dip galvanized object or the surface of the hot-dip galvanized object containing aluminum by immersion in the treatment solution or atomization using a sprayer, and a chemical reaction forms the coating. This is a method for forming a zinc phosphate film, which is characterized by forming a zinc phosphate film.

Among the treatment liquids used in the method of the present invention, the main liquid is an aqueous solution containing zinc phosphate and nitrite.

As the zinc phosphate component, phosphate-zinc Zn (H2PO4)2.2H20 or trizinc phosphate Zn3(PO4)-4H20 can be used. In the latter case, phosphoric acid is used in combination, but trizinc phosphate is not completely dissolved as it is, and a homogeneous solution is obtained only by the action of an auxiliary liquid, which will be described later.

Nitrite together with the auxiliary liquid promotes the reaction between the hot-dip galvanized material and zinc phosphate, making it possible to process at low temperatures near room temperature.

The sodium hydroxide in the auxiliary liquid dissolves and removes the aluminum component in the aluminum-containing hot-dip galvanized product, and the sulfuric acid radical functions to maintain the dissolved aluminum in a stable state.

In addition to the above-mentioned components, it is preferable to add a zinc ion source (such as ZnO) and phosphoric acid to the auxiliary liquid in order to suppress rapid reactions and improve initial compatibility.

As a result of a detailed comparative study of the appropriate concentration range of each treatment liquid, mixing ratio of main liquid and auxiliary liquid, and treatment time, with regard to processing capacity, difficulty in liquid management, and adhesion to the paint film, etc. The following concentration ranges and mixing ratios of the main liquid and auxiliary liquid were obtained.

Factory ash momme 1 Zn(H2PO4)・2H2゜25.0~75.0g or Zn 3(PO4) 2" 4H2026,1~78
．． 3g 83PO422,3-66,99 NaNO22-9g Dissolve the above mixture in 2.5 parts water.

When the zinc phosphate concentration is 10 g/i/or less, the film formed on the surface of the plated object is dense, but the formation rate is slow and requires a long treatment time. Moreover, when the amount exceeds 30 g/L, a tendency to form a rough film was observed.

118.2~153.7g or H2S04 (II sulfuric acid) 92.0~119.6m1 zno~2gH3Po
4 (75%) O ~ 20rd NaOH (20%) 325.0 ~ 425.0d H2O 100 ~ 250mj Contains dish main liquid 90-80 parts by weight auxiliary liquid
10-20 parts by weight This treatment solution does not contain any chemical solutions that require careful handling, such as chemical substances and nickel compounds, or require strict wastewater treatment equipment, as in the past. Another feature is that there is no

The standard steps for processing hot-dip galvanized materials and aluminum according to the present invention are as follows.

After immersing or spraying the treatment liquid at room temperature for 30 seconds to 2 minutes, it is washed with water and dried with warm air.

In principle, pretreatment is not necessary, but if the surface of the plated object is excessively contaminated with oil or rust, degreasing or rust removal may be necessary. In this respect as well, it is easier to handle the plated object and avoid contact with poisonous substances in the treatment solution, compared to the conventional method in which the treatment solution is dried while remaining on the surface.

It was generally believed that when a phosphate film is heated to 140° C. or higher, water of crystallization is rapidly lost, resulting in deterioration of corrosion resistance and adhesion to the paint film. However, the zinc phosphate film of the hot-dip galvanized product obtained by the present invention can be used in the fluidized dipping method for polyethylene resin, which requires high-temperature preheating of around 300°C, and vinyl chloride and nylon resin, which are heated at around 220°C. It was found that it maintains good adhesion to the paint film, has little performance deterioration due to heating, has excellent heat resistance, and can be used as a paint base. It is also considered that it can be sufficiently used as a coating base for electrostatic coating or powder electrostatic coating that is used at lower temperatures.

[Function] Basically, the zinc phosphate film obtained by the present invention is produced by the action of the auxiliary liquid, while preferentially dissolving and removing the layer containing concentrated aluminum near the surface of the hot-dip galvanized layer. This process forms a zinc phosphate film, which is essentially similar to the film produced by conventional methods.

It is believed that the following reaction occurs during the chemical conversion treatment of the zinc phosphate film of the present invention.

Main liquid reaction formula % Formula % Water-insoluble glue △ When making a bath using trizinc 7ate and phosphoric acid, the trizinc phosphate will not completely dissolve as it is, but a strong acidic auxiliary liquid will be mixed. (i.e., insoluble trizinc phosphate is dissolved by the action of phosphoric acid and sulfate).

At this point, the treatment solution containing trizinc phosphate and phosphoric acid and the solution containing phosphoric acid and zinc have been dissolved in the form of the same phosphate. Equation (2) is an equilibrium reaction equation in the treatment liquid without chemical conversion treatment, and although this reaction is reversible, the movement to the right is much faster than to the left. Equation (3) is the reaction equation between the processing solution and the surface of the plating layer when a plating object is added, and as a result, the parallel reaction in equation (2) moves to the right.
Subsequently, the reaction of formula (4) also moves to the right, forming a zinc phosphate film on the surface of the plating layer.

The layer containing concentrated aluminum and its oxides near the surface of the film is dissolved and removed by sodium hydroxide as shown in equation (5), and then removed by aluminum sulfate according to equation (6). It is stably retained in the processing solution as a salt. These reactions proceed simultaneously at room temperature. In the conventional method, aluminum is dissolved and removed by the action of a carbide, so the treatment temperature is relatively high, the stability of the treatment liquid is poor, and the treatment capacity is extremely small. The reaction formula of the treatment liquid when it contains a compound is:
It is generally thought that as the chemical reaction progresses, the amount of dissolved aluminum increases, and A gradually increases in the processing solution.
Complex fluoride salts of aluminum such as F are generated and inhibit the film formation reaction.

Al3+ +3 F-→A force F (8) The present invention is capable of dissolving and removing aluminum concentrated near the surface of the plating layer and forming aluminum complex salts in the processing solution, as seen in the reaction of the auxiliary solution. , is characterized in that it is carried out using different auxiliary liquid components.

[Example] Next, examples of the present invention will be shown.

Preparation of treatment liquid [I] Main liquid Main liquid A and main liquid 13 having the following compositions were prepared using trizinc phosphate and zinc phosphate as zinc phosphate.

Main liquid A Zn 3 (Po 4> 24H20 21,79 H3po 4 18.6gNaNO21
, 29 H201J2/ Main liquid B Zn (t-12P04) 2'' 2H2021, 09 NaNO21, 29 H201j!/ [n] An auxiliary liquid was prepared by blending the components listed in the supporting list.

CH2SO4105m1 zn0 1.89H3P04
(75%) Add 4.3 d of NaOH (>76.0 g as a 20% aqueous solution) to make a total volume of 695 d.

Examples 1 to 4 Hot-dip galvanized products containing aluminum were prepared from the above-mentioned main liquid A and main liquid B and an auxiliary liquid without any degreasing or surface activation treatment as when they were placed, Table 1 4 shown in
A zinc phosphate film was formed using a different treatment solution under the dipping conditions described in the notes.

Table 1 Note) Pre-treatment: None Treatment temperature: Two room temperature Treatment time: 90 to 120 seconds for Examples 1 and 3, 30 to 90 seconds for Examples 2 and 4 Post-treatment: After washing with tap water (1 minute), air drying Examples 5 to 8 According to Examples 1 to 4, zinc phosphate films were formed using the four types of treatment solutions shown in Table 2 under the spray conditions noted.

Table 2 Note) Pre-treatment: None Treatment temperature Two room temperature Treatment time: 90 seconds for Examples 5 and 7 60 seconds for Examples 6 and 8 Post-treatment: After washing with tap water (1 minute), air drying The hot-dip galvanized products containing aluminum of Examples 1 to 8 were subjected to fluidized immersion treatment with polyethylene resin under the following coating conditions.

Preheating temperature: 280℃, 10 minutes Immersion time: 7.5 seconds Paint: Polyethylene resin Coating film thickness: Approximately 400 μm After painting, make 10 equal increments vertically and horizontally on the surface of the plating layer according to the base grain test (100 sections). Table 3 shows the adhesion to the coating film determined as the residual percentage of the coating film. At the same time, for comparison purposes, the results obtained using an improved zinc phosphate film treatment solution containing a commercially available abutment are shown.

Table 3 As is clear from the table, the film formed by the method of the present invention has a very good rate of bacteria remaining on the film, and is particularly superior to the conventional improved treatment solution in that there is almost no effect of change over time. Much better than by.

[Effects of the Invention] As explained above, the present invention is a method that can form a zinc phosphate film extremely easily at room temperature on the surface of a hot-dip galvanized product and a hot-dip galvanized layer containing aluminum, and is similar to a fluidized dipping method. It is suitable as a base treatment for coating methods that require high-temperature heating of objects, and is effective in significantly improving adhesion to paint films. It is also sufficiently effective when used as a base for ordinary painting.

Claims (1)

[Claims] 1) Molten zinc characterized in that it is treated with a mixed aqueous solution of a main liquid consisting of zinc phosphate and nitrite and an auxiliary liquid with a pH of 2 or less containing sulfuric acid radicals and sodium hydroxide as main components. A method for forming a phosphate film on plated objects. 2) The method for forming a phosphate film according to claim 1, wherein the hot-dip galvanized product is an aluminum-containing hot-dip galvanized product. 3) The phosphorus according to claim 1, wherein the main liquid is an aqueous solution containing 10 to 30 g/l of zinc phosphate in terms of Zn(H_2PO_4)_2.2H_2O and 3 g/l or less of nitrite in terms of NaNO_2. Acid film formation method. 4) The method for forming a phosphate film according to claim 1, wherein the auxiliary liquid is an aqueous solution containing 50 to 65 g/l of sulfate radicals and 65 to 85 g/l of sodium hydroxide as main components. 5) The method for forming a phosphate film according to claim 1, which uses a mixed aqueous solution of 80 to 90 parts by weight of the main liquid and 10 to 20 parts by weight of the auxiliary liquid. 6) The phosphate film forming method according to claim 1, wherein the auxiliary liquid further contains a zinc ion source and/or phosphoric acid. 7) The method for forming a phosphate film according to claim 1, wherein the treatment is immersion or spray treatment. 8) The method for forming a phosphate film according to claim 1, wherein the treatment is performed at room temperature for less than 2 minutes.