JPS60593B2 - Method for preventing groove-like corrosion of ERW steel pipes in circulating water piping systems - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention aims to prevent groove-like corrosion (hereinafter referred to as
This article relates to a method for preventing groove corrosion (referred to as groove corrosion).

Generally, it was thought that sufficient durability could be ensured even if inexpensive carbon steel electric resistance welded pipes were used for piping systems for relatively weakly corrosive industrial water, tap water, and the like.

However, in recent years, it has become clear that even under such mildly corrosive usage conditions, the port reinforcement section of the main pipe is subject to early groove corrosion, eventually leading to water leakage. The main cause of such groove corrosion is that the natural potential of the welded part of electric pipes is lower than that of the surrounding base metal, so when used as piping, the welded part becomes the anode and the base metal becomes the cathode. It is thought that a type of battery is formed and the welded part of the anode is locally selectively corroded.

Therefore, it is possible to suppress groove corrosion by eliminating the potential difference between the welded part and the base metal part. One of the biggest causes of this potential difference is the difference in metal structure between the two. That is, since only the welded portion is heated and then rapidly cooled during pipe manufacturing, the structure thereof is a quenched structure of martensite or pennite, whereas the matrix portion is a ferrite + pearlite structure. Conventionally, as a method to prevent groove corrosion of a welded pipe by eliminating this structural non-uniformity, welding is performed by subjecting the entire pipe to welding (for example, holding it at a temperature of around 950 qo for 10 minutes or more and then cooling it in air). There is a method of changing the structure of the part from bainite and martensite to ferrite + pearlite.

However, if such a competitive process is added to the manufacturing process of electric resistance welded pipes, the productivity will decrease accordingly, which is not desirable. In addition to this heat treatment method, Cu, Cu,
There is a method to prevent groove corrosion in welds by adding alloying elements such as Ti and Sb in combination to make the wire mesh higher quality.
This method will inevitably increase the cost, and the original benefits of Kamesokan will be diminished.

SUMMARY OF THE INVENTION The present invention aims to provide a method for avoiding a decrease in productivity and an increase in cost of electrical resistance welded pipes as much as possible, and effectively preventing groove corrosion of electrical resistance welded pipes for piping systems. Conventionally, in piping systems where industrial water or tap water circulates, corrosion inhibitors (hereinafter referred to as inhibitors) such as polymeric phosphates, organic substances, or silicate-based substances are mixed into the circulating water. It is generally practiced to suppress the corrosion of

Inhibitors are originally used for the purpose of suppressing general corrosion of piping, and it is well known that inhibitors alone have no significant effect on groove corrosion. However, according to the experiments and research conducted by the present inventors, under the conditions in which this inhibitor is used, the heat treatment time is on the order of seconds, which is much shorter than that of ordinary weaving, at the welded part of the silk weaving pipe and its vicinity. It has been found that groove corrosion can be effectively prevented by a synergistic effect with the inhibitor by simply applying local heat treatment. That is, the present invention constructs a piping system using a carbon steel electric resistance welded pipe that has undergone local heat treatment in which the welded part and its vicinity are maintained at 900°C or higher with two to two sand gaps and then left to cool, and the circulation of this piping system is It is characterized by using an inhibitor mixed in water. In addition, the above-mentioned carbon steel is CO. 3%
Below, Sio. 30% or less, Mno. 25-1.00%
, P.O. 040% or less, SO. 0.03% or less, with the remainder essentially consisting of Fe.

Using an inhibitor alone or performing short-term local heat treatment as described above has little effect on preventing groove corrosion in open seam pipes that make up piping, but it is recommended that both operations be performed at the same time. When done, they work synergistically and exhibit excellent effects in preventing groove corrosion.

Moreover, since this method requires only an extremely short heat treatment time in the manufacturing stage of the silk-sewn tube, there is no significant drop in productivity and the cost is low. Next, the heat treatment conditions in the method of the present invention will be described.

The reason why the heat treatment temperature was set at 90,000 or higher is that if the temperature is lower than 900 C, the hardened structure of the weld zone, that is, bainite or martensite, cannot be brought close to the ferrite + pearlite structure of the base metal in a short time, so an inhibitor is used. We cannot expect any synergy at the bottom.

On the other hand, as for the heating time, it is impossible to bring about a structural change if the heating time is less than 2 seconds, so it is necessary to set the heating time to 2 seconds or longer. is necessary.

As the inhibitor, currently available commercially available polymeric phosphate, organic or silicate inhibitors can be used, and the inhibitor is preferably used at a concentration of about 1 to 1000 ppm.

Next, the effects of implementing the present invention will be described. The welded parts of carbon steel electric resistance welded pipes having the components shown in Table 1 were subjected to heat treatment according to the method of the present invention under the conditions shown in the table, and this was treated with circulating water containing an inhibitor (20 PMCI-, 7.0, temperature 50°C). The pipe was used for 80 days at a flow rate of 1.0° C. and a flow rate of 1.0/sec).

Table 1 shows the groove-like corrosion of this hex-sewn pipe.

In addition, for comparison, carbon steel electrical pipes were used as they were, or heat treated in a way that was not compatible with the method of the present invention, and then used under the same conditions as above or without using an inhibitor, and the groove corrosion status of the pipes was investigated. The results are also listed in the same table. The evaluation of corrosion status is divided into three types: 0: Hardly occurs (corrosion depth 20 mm or less), △: Slightly occurs (20 to 150 mm), ×: Significantly occurs (150 mm or more). Yes. 1st
Table *Types of inhibitors A: Polymerized phosphate type B: Organic substance type 0: K-- In the above table, of course, if heat treatment ■ and inhibitor addition ■ are not performed at all 6), ■
Significant groove corrosion occurred even in cases 7) to 11) where operations 1 and 2 were performed alone, and slight improvement was observed in cases 12) to 14) where the heat treatment conditions were outside the scope of the present invention even when operations 1 and 2 were used together. However, when the method of the present invention was implemented 1)
-5) all show extremely good results.

As explained above, the method of the present invention exhibits an excellent effect as a measure to prevent groove corrosion of the threaded pipes that constitute the circulating water piping system without impairing the original economic efficiency and productivity of the open pipes, and at the same time Since it uses an inhibitor, it has the rationality of suppressing general corrosion, and can be said to be an extremely useful invention in terms of maintenance management of piping systems.

Claims (1)

[Claims] 1 The piping system is constructed of carbon steel ERW pipes that have undergone local heat treatment in which the welded part and its vicinity are held at 900°C or higher for 2 to 20 seconds and then left to cool, and polymerized phosphate is added to the circulating water of the piping system. 1. A method for preventing groove-like corrosion of electric resistance welded steel pipes in a circulating water piping system, characterized by using a corrosion inhibitor such as a type, an organic substance type, or a silicate type.