How to avoid common problems such as coating delamination in construction?
In FBE plastic-coated steel pipe anticorrosion engineering, coating delamination, debonding and pinhole are the core pain points that affect the construction quality. These problems seem to be accidental, but in fact, they mostly stem from the omission of process control, improper material adaptation or environmental impact. Once they appear, they will not only increase the cost of rework, but also shorten the service life of pipelines and even cause safety hazards. In this paper, the causes and avoidance methods of common problems in dismantling coatings are discussed in the form of questions and answers, which provides professional reference for quality control in construction.
What is the core cause of coating delamination?
Coating delamination is the most typical problem in FBE plastic-coated steel pipe construction, which is mainly manifested in peeling cracks between coating and steel pipe surface and multi-layer coating. The core causes can be summarized into three categories. First of all, the thermal stress is unbalanced, and the thermal expansion coefficient of FBE coating and steel is quite different. Residual stress will be generated during heating and cooling during construction. If the stress cannot be effectively released, the bonding force between coating and substrate will be destroyed.
Secondly, the adhesion is insufficient, which is directly related to the substandard surface treatment. The residual scale, oil stain and moisture on the steel pipe surface will hinder the chemical bonding between the coating and the substrate, leading to the decrease of adhesion and easy delamination in the later stage. In addition, environmental factors can't be ignored. If the pipeline is exposed to humid and corrosive environment for a long time after construction, it will cause electrochemical corrosion, and the corrosion products will further aggravate the coating peeling and form a vicious circle.
Besides stratification, what other high-frequency quality problems are there?
In addition to coating delamination, there are three high-frequency problems in FBE plastic-coated steel pipe construction: pinhole, crack and sag. Pinholes are mostly caused by insufficient preheating, and the residual water on the surface of steel pipe vaporizes during high temperature spraying, forming tiny pores, which will become infiltration channels of corrosive media and reduce the anti-corrosion effect.
The generation of cracks is related to improper cooling rate. Too fast cooling will cause stress concentration in the coating, especially in the complex parts such as welds, where fine cracks are easy to appear and gradually expand. Sagging is common in the construction of small-diameter pipelines. Because of unreasonable spraying parameters and uneven coating thickness, the powder flows along the surface of the pipeline after melting, forming a local coating that is too thick or thin, which affects the consistency of protection. Although these problems have different forms, they are closely related to the construction process control.
What preparations should be made before construction to avoid risks?
To avoid coating quality problems, it is necessary to build a defense line from the preparation link before construction. The first task is to strictly control the quality of materials, select FBE epoxy powder conforming to SY/T 0315 standard, ensure the uniform particle size of the powder and reach the melting index standard, and at the same time, check the surface of the steel pipe for no scratches, depressions and other defects, so as to avoid the substrate problems from affecting the coating adhesion.
Secondly, optimize the construction scheme, adjust the process parameters according to the pipeline diameter and working environment, define the key indicators such as preheating temperature, spraying pressure and cooling rate, and formulate special construction procedures for the corners of welds and bends. In addition, it is necessary to check the construction environment in advance to avoid construction under the condition that the humidity is higher than 85% and the temperature is lower than 5℃, and at the same time, do a good job in calibrating the construction equipment to ensure the accurate operation of the shot blasting machine, spraying equipment and temperature measuring instrument.
How to accurately control key links in construction?
Fine control of construction process is the core to avoid problems such as stratification. In the surface treatment, shot blasting and derusting technology should be adopted to control the surface grade of steel pipe at Sa2.5 level and the roughness at 40-80μm, and spraying should be completed within 2 hours after treatment to prevent secondary oxidation of the surface. In the preheating step, the uniform temperature rise is realized by the intermediate frequency heating device, the temperature is stabilized at 180-220℃, and the deviation is controlled within 5℃, so as to completely remove the residual moisture.
When spraying, electrostatic spraying technology is adopted to optimize the spray gun angle and moving speed to ensure uniform coating thickness. The construction of multi-layer coating needs to grasp the "golden window period" of 30 seconds and complete subsequent coating before FBE coating is completely cured. The cooling link adopts gradual cooling to avoid stress caused by excessive temperature difference, and it is forbidden to stack directly after cooling to prevent mechanical damage of the coating.
How to deal with quality problems scientifically?
If problems such as coating delamination and pinholes are found during construction, they should be dealt with in time to avoid hidden dangers. For the local layered area, the damaged coating should be thoroughly removed, and the surface treatment, spraying and curing should be carried out again. The treatment range should exceed the damaged area by more than 50mm to ensure that the repaired part is closely connected with the original coating.
Pinhole defects can be repaired by applying special epoxy coating. Before repairing, impurities around pinholes should be cleaned, and the thickness of the coating should not be lower than the original coating standard. If the crack or delamination range is large, it is necessary to cut the damaged pipeline section, weld it again and coat it as a whole. After disposal, it needs to be verified by adhesion test, leak detection and other means to ensure that the repair quality meets the standard.
To sum up, to avoid the coating problem in FBE plastic-coated steel pipe construction, it is necessary to run through the whole process of "prevention in advance, control in the process and disposal afterwards". Only by strictly controlling the quality of materials, optimizing process parameters and accurately controlling key links can problems such as delamination and pinholes be reduced from the source and the anticorrosion performance and service life of pipelines be guaranteed.
