JPH04180867A - Application of epoxy resin powder to steel pipe - Google Patents

Abstract

PURPOSE:To lower the inner quality defects and obtain a coating with good mechanical functions by applying a powdery epoxy resin painting while the surface of a steel pipe is preheated after chromate chemical conversion treatment, and carrying out post-heating at the temperature lower than the thermal decomposition temperature after gelling of the epoxy resin. CONSTITUTION:After the surface of a steel pipe is cleaned by blast treatment, etc., and chemical conversion with chromate is carried out, the surface of the steel pipe is preheated to 170-210 deg.C and a powdery epoxy resin paint is applied and post-heating is carried out at the temperature lower than the thermal decomposition temperature of the epoxy resin after the applied epoxy resin gels. In this way, since the preheating temperature of the steel pipe is kept low and the period when the epoxy resin paint is melted without being hardened by heat is made long, the time becomes enough to let bubbles owing to ambient air mixed to the paint during the painting float up and the defects of the internal quality are remarkably lowered. Also, since chemical conversion with chromate is carried out before the preheating of the steel pipe, adhesion strength of the epoxy resin powder, that is cathodic peeling resistance of the coating, is not deteriorated even if the preheating temperature is lowered.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for coating steel pipes with epoxy resin powder.

<Prior art> When applying powdered epoxy resin to the surface of a steel pipe, the surface of the steel pipe is usually preheated to 218 to 260'C (hereinafter referred to as preheating), and then powdered epoxy resin paint is applied on top of this. , Utilizing the sensible heat of the steel pipe during the natural cooling process of the steel pipe,
A well-known method is to melt, fluidize, and harden powdered epoxy resin paint (Steel Research No. 314 (1)).
(984) P, 111-112').

Epoxy resin is a thermosetting resin, and if the curing reaction is insufficient, the coating film will be brittle and mechanical performance such as bending resistance will deteriorate. A method is taken to prevent insufficient curing.

However, if the coating temperature is higher than necessary, the melting time of the powder epoxy resin paint will be shortened, and it will harden with air bubbles trapped inside, causing internal defects such as forming a paint film. , the coating surface hardens without becoming smooth, resulting in an appearance defect called orange skin.

As mentioned above, two methods exhibiting contradictory trends regarding coating temperature.
It is difficult to simultaneously satisfy two quality characteristics: hardening of the paint film, internal quality defects, and poor appearance.In reality, emphasis is placed on hardening of the paint film, and fleshy defects are dealt with later in the inspection process. The method used was to detect and repair defects using a pinhole test.

<SB to be Solved by the Invention> In the conventional technology, it is difficult to eliminate the above-mentioned flesh defects, and the number of man-hours required to repair the flesh defects detected by the pinhole test is also large.

An object of the present invention is to provide an epoxy resin powder coating method for steel pipes that can reduce internal defects to a practically negligible level and obtain a coating film cured to a level with good mechanical performance. do.

<! Means for Solving Problem 18> The present invention cleans the surface of the steel pipe by blasting or the like and performs a chromic acid-based chemical conversion treatment, and then the surface of the steel pipe is
Epoxy for steel pipes, which is characterized in that the steel pipes are preheated to 210"C, coated with powdered epoxy resin paint, and after the epoxy resin coated on the surface of the steel pipes gels, they are post-heated at a temperature below the thermal decomposition temperature of the epoxy resin. This is a resin powder coating method.

<Function> According to the method of the present invention, the preheating temperature of the steel pipe is lowered and the time during which the epoxy resin paint is melted without being cured by heat is increased, so that when spraying the paint, there is no There is sufficient time for bubbles caused by air in the mixed atmosphere to float, and internal defects are significantly reduced.

The preheating temperature was limited to 210'C or less.
This is because if it exceeds C, the effect of reducing flesh quality defects will be weakened.

Further, the reason why the temperature is limited to 170°C or higher is because the cathodic peeling resistance of the coating film deteriorates when the temperature is lower than 170''C.

Furthermore, the post-heating temperature may be any temperature as long as the coating film is cured, but if the epoxy resin thermally decomposes, the mechanical performance and corrosion resistance will deteriorate significantly, so it is limited to a temperature below the thermal decomposition temperature of the epoxy resin. .

Furthermore, according to the method of the present invention, the chromic acid-based chemical conversion treatment is performed before preheating the steel pipe, so even if the preheating temperature is lowered, the adhesion of the epoxy resin powder, that is, the cathodic peeling resistance of the coating film, is not damaged.

<Example> Using a steel pipe with an outer diameter of 800-1 and a wall thickness of 15 m, powder epoxy resin was electrostatically coated on the outer surface of the pipe to a film thickness of 400 am. Table 1 shows the various conditions and evaluation results at this time.
Shown below.

First, blasting treatment and chemical conversion treatment were performed as pre-painting treatments. Specifically, the outer surfaces of all the steel pipes were blasted using steel glint, and the degree of polishing was adjusted to Swedish standard SI S 05-5900 Sa3.0. Thereafter, a chromate-based chemical conversion treatment agent was applied to the surface of the steel pipe to form a chromate film with a total chromium deposition amount of 0.3 g/bo. However, some comparative examples do not undergo chemical conversion treatment.

Thereafter, the steel pipe was heated using an induction heating coil, and immediately after reaching the peak temperature, powdered epoxy resin was sprayed and painted while it was being left to cool naturally. The surface temperature of the steel pipe at the time of coating is defined as the preheating temperature, and the decrease from the peak temperature is about 5°C. Thereafter, when performing post-heating, the steel pipe was heated again with an induction heating coil, allowed to cool naturally until the coating film on the surface of the steel pipe reached 100°C, and then water-cooled to room temperature. The peak temperature during this post-heating is defined as the post-heating temperature. In addition, when post-heating was not carried out, the coating was allowed to cool naturally to 100° C. after coating, and then water-cooled to room temperature. These heating patterns are schematically shown in FIG.

Next, a method for evaluating the coating film will be explained.

First, in the pinhole test, 10
By applying a voltage of ν to
Evaluation was made as the number per 0.

The degree of hardening of a coating film is determined by determining the glass transition temperature T g + by DSC analysis of the coating film, and then determining the difference ΔTg between the glass transition temperature Tgt after heating the coating film to 280°C in a laboratory.
= I Tg+ Tgz l was calculated and evaluated.

- For boat fishing, we judge that the coating film has hardened when ΔTg≦3”C, and this judgment criterion was also adopted this time.

The bending performance of the coating film was determined by cutting out a piece of steel from a steel pipe with the coating film attached, bending it so that the surface strain of the coating film was 4.0%, and visually determining the presence or absence of cracks.

For evaluation of corrosion resistance, the steel surface was held at a potential of 5 µm against a calomel electrode in a 3% NaC/molten solution at 70°C for 10 days, and the peeling length was evaluated. .

In the present invention example shown in Table 1, Nnl is 2.3, flesh defects are reduced due to the reduction in preheating temperature, and the coating film is sufficiently hardened due to the effect of postheating, showing good bending performance. In addition, good results were obtained in terms of cathode peeling resistance.

Comparative Examples A to E are coating methods conventionally used for boat fishing, but since the preheating temperature is higher than that of the present invention, the occurrence of internal quality defects is unavoidable.

In addition, when chemical conversion treatment is not performed as in Comparative Example H, the cathodic peeling resistance is poor, and especially when the preheating temperature is low as in Comparative Example, high temperature and
The level is unsuitable for use in a humid environment. This is said to be because when the preheating temperature is low, the viscosity of the molten epoxy resin is low and it becomes difficult to adapt to the steel surface.

Nfll, 2.3 of the present invention is a cathode resistant material that suppresses the peeling reaction at the interface between the coating film and the steel by lowering the preheating temperature and applying chemical conversion treatment to significantly reduce internal defects. It is characterized by suppressing deterioration in peeling performance. Therefore, in the case of Comparative Example F, in which the chemical conversion treatment was not performed using the same heating pattern as Invention Examples No. 2, both internal defects and bending performance were good, but the cathode peeling resistance was poor. ing.

However, in the case of Comparative Example G in which the preheating temperature was lowered too much, there were no internal defects, the coating film was cured, and the bending performance was good, but the cathodic peeling resistance was poor. This is because the chemical conversion treatment could not fully compensate for the aforementioned decrease in cathode peeling resistance during low-temperature preheating.

Next, Comparative Example H is an example in which post-heating is omitted from Invention Example Nα2, but the hardening is insufficient and the bending performance is inferior. Another aspect of the present invention is that curing is accelerated by applying post-heat to a coating film that is insufficiently cured due to low-temperature preheating.
There are two characteristics.

However, if the post-heating temperature exceeds the decomposition temperature of the epoxy resin (approximately 280° C.) as in Comparative Example I, even if hardening is achieved, the bending performance will be poor.

<Effects of the Invention> As explained above, the present invention provides a powder epoxy coated steel pipe that significantly reduces the occurrence of internal defects and has excellent bending performance and cathodic peeling resistance. be able to.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph schematically illustrating the heat exchangers of Examples of the present invention and Comparative Examples. Rin - Dome Sashise

Claims (1)

[Claims] After cleaning the steel pipe surface by blasting or other means and performing chromic acid-based chemical conversion treatment, the steel pipe surface is preheated to 170-210°C and powdered epoxy resin paint is applied. A method for coating steel pipes with epoxy resin powder, which comprises gelling and then post-heating at a temperature below the thermal decomposition temperature of the epoxy resin.