JPS6038464B2 - Rust prevention treatment method for steel materials - Google Patents

Description

[Detailed description of the invention] TECHNICAL FIELD The present invention relates to an improved rust prevention treatment method for steel materials.

As part of the chemical cleaning of boilers, heat exchangers, and other steam generation plants, etc., after acid cleaning, they are washed with water to discharge the acid solution, and finally, the steel surface is protected by rust prevention treatment, as shown in Table 1. Conventional rust prevention treatments have the following drawbacks.

In the treatment method shown in Table 1a, the treatment liquid contains citric acid and hydrazine, which are COD components, so the COD in the wastewater of this method is approximately 80 pml, which is the emission standard value of each prefecture. It far exceeds 10-25 ppm.

Therefore, waste liquid treatment to remove COD is essential.
The processing requires a lot of effort and cost. b The treatment method shown in ■ also contains citric acid and nitrite, which are COD loading components, in the treatment solution, resulting in a COD of approximately 110 pm, which, like ■, far exceeds the emission standard value. Processing requires a lot of effort and cost.

In order to provide a rust prevention treatment method for steel materials that does not have these drawbacks, the present inventors have been conducting research on rust prevention solutions that do not contain COD components. The present invention was achieved by discovering that a solution whose pH was adjusted by adding a combination of the following is a rust preventive solution suitable for this purpose.

That is, the present invention uses at least one member of the group consisting of sodium pyrophosphate, sodium tripolyphosphate, sodium tetrapolyphosphate, and sodium hexametaphosphate in an amount of 0.
05~0. A solution whose pH is adjusted to 2.5 to 6.5 by adding phosphoric acid to an aqueous solution containing 30oo to 80% by weight.
The present invention relates to a rust prevention treatment method for steel materials, which is characterized by treatment with oo.

The reason why the blending amount, appropriate pH, and temperature of the rust prevention treatment liquid in the present invention are defined as described above is as follows.

○} Automobile phosphate 0.05-0. $wt% At least one of sodium pyrophosphate, sodium tripolyphosphate, sodium tetrapolyphosphate, and sodium hexametaphosphate is used as the main agent for forming a rust-preventing film of iron phosphate. It is related to the liquid ratio (treatment liquid amount) / (treated area of steel material) and the amount of lacquered iron ions during anti-corrosion treatment, and the liquid ratio during anti-mirror treatment of actual cans is approximately 1.5. If the amount of iron ions is about 10 pm, then 0
．． If the content is 0.05% by weight, iron precipitation may occur, and the amount of formation of the rust-preventing film is small (the rust-preventive power is also weak, and if the content is 0. If the amount exceeds 3% by weight, problems such as decrease in the polymerized phosphate due to hydrolysis and economic efficiency will occur, so this is set as the upper limit.

■ pH range 2.5 to 6.5 Phosphoric acid is used as a pH adjuster, but if the pH is lower than 2.5, corrosion of the steel surface subjected to rust prevention treatment will increase, and the delicate phosphate iron Rust formation becomes difficult and p
Chemical formation of the iron phosphate rust preventive film is not preferable if the temperature exceeds H6.5.

'3 - Temperature 30-80qo The appropriate temperature during rust prevention treatment depends on the pH of the treatment solution, so around 30-40℃ is preferable when the pH is around 2.5.
A pH of 6.5 and 60 to 80 is appropriate.

If the temperature exceeds 80°C, the hydrolysis of the polymerized phosphate will increase, and insoluble matter will adhere to the surface of the steel material, which may result in a lack of delicateness of the coin protection membrane, which is undesirable. This is not preferable because the rate of chemical formation of the anticorrosion film becomes slow.

Although the mechanism by which the rust preventive solution of the present invention forms a rust preventive film on the surface of steel materials is not clear, it is thought to be due to the following chemical action.

In other words, since free phosphoric acid exists in the rust preventive solution, the phosphoric acid reacts with the surface of the steel material to form monoferrous phosphate [Fe
(2P04)2], and this ferrous phosphate has the property of dissociating on the surface of the steel material as shown in the following formula [1', {2] at an appropriate pH concentration and temperature. Fe(QP04)2
→FeHP04 Toka 3P04 {1)*eH
P04 → Fe3 (P04) 20th 3P04
{2}{1}, ■Iron phosphate [Fe
HP04] and ferric phosphate [Fe3(P04)2] are both insoluble, so they crystallize and precipitate on the surface of the steel material at the same time as they are formed.

As mentioned above, ferrous phosphate is generated by a chemical reaction between free phosphoric acid and the steel surface, so at the interface between the steel surface and the treatment solution, the pH value of the solution increases (the concentration of free phosphoric acid decreases). As a result, the dissociation of equations {1} and '2) newly proceeds in the right direction, so insoluble diferric phosphate and tertiary iron phosphate gradually develop on the steel surface, and a rust preventive film is chemically formed. be done. The method of the present invention provides the following effects.

'1} Since rust prevention treatment of steel materials can be performed using polymerized phosphate and phosphoric acid that do not contain COD components, waste liquid treatment requires only simple neutralization, resulting in significant savings in waste liquid treatment costs.

[2] The appropriate pH range during rust prevention treatment is wide, and since neutralization is not required after chemical conversion treatment, rust prevention treatment management is easy and treatment time is shortened.

The method of the present invention is suitable as a temporary rust prevention treatment method after acid cleaning of boilers, heat exchangers, etc. in thermal power plants, chemical plants, etc.

Example 1 For 100 rust preventive solutions prepared as shown in Table 2,
Two strip-shaped test pieces (5 specific ribs x 25 ribs x 5 sides) of boiler steel pipe materials STB42 and STBA24 were immersed for 2 hours under the temperature and pH conditions shown in Table 2. When the chemical formation state was investigated, it became as shown in the right column of Table 2.

This column indicates the state of rust prevention film formation: ◎ is a glossy and delicate film with strong rust prevention ability, 0 is a thin film with weak rust prevention ability, and △ is a rough film with large corrosion on the steel surface. The rust prevention ability is weak,
× indicates that the anticorrosive film is hardly chemically formed. As can be seen from this table, test number 1 in the claims
In Nos. 0 to 21 and Nos. 26 to 27, a glossy, delicate iron phosphate antirust film with strong antirust ability was chemically formed on the steel surface.

Claims (1)

[Claims] 1 At least one member of the group consisting of sodium pyrophosphate, sodium tripolyphosphate, sodium tetrapolyphosphate, and sodium hexametaphosphate at a concentration of 0.05 to 0.
Phosphoric acid was added to an aqueous solution containing 3% by weight to pH 2.5.
A rust prevention treatment method for steel materials, characterized by treating at 30°C to 80°C with a solution adjusted to a temperature of 6.5°C.