Corrosion of stainless steel pipeline in offshore environment
In an island power transmission project: 321 stainless steel seamless pipe is used for water system transmission. The surface treatment method of outdoor pipeline is sand blasting. After being put into use for 10 months, it was found that the surface of the outdoor pipeline was seriously rusted and there were more rusts. It is found that the rust on the surface of the pipeline is floating rust, which can be removed by wiping with a drawcloth. In order to find out the cause of corrosion of outdoor stainless steel pipeline in a short time and solve the problem of service life of stainless steel pipeline. In this paper, the corrosion causes of stainless steel pipeline in complex marine environment such as high temperature, high humidity, high salt sea breeze, salt fog and sea water splash are analyzed comprehensively. The mechanism of chloride ion corrosion damage to stainless steel pipeline surface in high humidity sea breeze and salt fog is analyzed emphatically, and some anti-corrosion measures are put forward to provide effective solutions for the implementation of similar projects in the future.
Comprehensive analysis of corrosion causes
Chemical composition analysis of pipeline
The chemical composition of stainless steel pipe was analyzed by material analyzer. The results show that the chemical composition of the pipe meets the requirements of 321 brand.
Corrosion analysis of stainless steel pipeline in marine environment
There are many differences between the marine atmospheric environment and the inland atmospheric environment. When analyzing the corrosion of stainless steel pipes, it is necessary to consider many factors such as high temperature, high humidity, high salinity sea wind, salt fog, sea water splash and so on.
High salinity sea breeze, salt fog and sea water splash
In the marine environment, the sea wind and waves are frequent. The waves caused by the sea wind make a large number of salt liquid in the sea water mix with the atmosphere, so that there are a large number of salts in the atmosphere, and there are a large number of chloride ions in these salts. The chloride ions in these salts have a high conductivity, which is easy to form electrolyte solutions, and form micro batteries on the metal surface, which enhances the corrosion activity and destroys the metal surface A passive film on a bad metal surface.
Under the condition of high temperature in the marine environment, a large number of seawater will produce atomization phenomenon, forming a large number of salt fog, and the main components of salt fog are similar to seawater, which will also produce corrosion on the surface of stainless steel pipes. The main cause of corrosion is the large amount of chloride ions in salt spray. Electrochemical corrosion is the main form of corrosion of stainless steel pipe surface by salt spray. The electrochemical corrosion process of salt spray on stainless steel pipes is as follows: there are a lot of sodium ions and chloride ions in the salt spray, and the corrosion of the salt spray on the surface of stainless steel pipes is caused by a lot of chloride ions; chloride ions have a strong penetration ability, which can easily penetrate the oxide layer and protective layer on the metal surface, and electrochemical corrosion is formed by electrochemical reaction with the metal interior. When the salt mist settles on the surface of the stainless steel pipe, there is a large amount of water vapor in the salt mist. First, a salt water film is formed to cover the surface of the stainless steel pipe. At this time, the surface of the stainless steel pipe is equivalent to being immersed in the sodium chloride solution containing oxygen. The surface of the stainless steel pipe will form two poles of the primary battery, causing corrosion. Therefore, salt spray corrosion and salt water corrosion are different in mechanism, and salt spray corrosion is higher.
High temperature and humidity environment
The temperature and humidity in the marine environment are much higher than in the inland environment. The high temperature and humidity environment makes the evaporation speed of seawater faster, and the evaporation of a large amount of water vapor has a great impact on the corrosion of stainless steel pipes. As a good medium, water vapor intensifies the transmission speed of various ions and acts as a good corrosion medium. With the large amount of chloride ions in the carrier water penetrating into the pipeline, electrochemical corrosion or chemical corrosion occurs
At the same time, the erosion of the waves makes the rusty rust layer fall off and accelerates the progress of corrosion. The growth of marine organisms on the surface of stainless steel pipeline can seriously damage the protective layer of original objects (such as the passivation film on polished pipeline surface), which causes the pipeline to be corroded. Meanwhile, the metabolites of marine organisms (including sulfide) make the corrosion environment of stainless steel pipeline further deteriorated, which leads to the aggravation of corrosion. If the area where the pipeline is located is acid rain area, the corrosion rate of the pipeline is also proportional to the pH value of acid rain.
Anticorrosive measures for stainless steel pipeline
Start from the pipe material, and strictly control the quality of raw materials.
Strictly inspect the stainless steel pipes and pipe fittings. To ensure that the material meets the requirements. In terms of material selection, considering both economy and function, under special operation conditions, austenitic stainless steel, duplex stainless steel, titanium and titanium alloy, glass fiber reinforced plastic, etc. with stronger corrosion resistance, can be selected.
When selecting materials, the corrosion allowance of pipes and fittings shall be increased appropriately to meet the service life requirements of the pipeline system.
Surface coating protection
At present, for the corrosion of stainless steel pipeline in the environment, in addition to the selection of materials with strong corrosion resistance, the corrosion resistance can also be improved by coating on the surface of stainless steel pipeline. In view of the problem that the surface of stainless steel pipeline base material is relatively smooth and the conventional anticorrosive coating has poor adhesion on its surface, an effective anticorrosive coating for stainless steel in marine environment, epoxy powder coating, has been developed. The coating is a new type of coating which is completely free of solvent and is sprayed into film in the form of powder.
The electrochemical protection methods mainly include protector protection, cathodic protection and anode protection.
- (1) The protection method of protector refers to connecting the metal with low electrode potential and the metal to be protected to form the primary battery. The metal with lower electrode potential is dissolved as anode, and the protected metal is used as cathode to avoid corrosion.
- (2) The cathodic protection method is to connect the protected metal to the negative electrode as a cathode by adding a DC power supply, so that the potential of the protected metal can be adjusted to a negative direction and enter the non corrosive area of the E-pH diagram; connect the waste metal to the positive electrode as an anode to corrode the waste metal and protect the cathode.
- (3) The anode protection method is to connect the protected metal to the most anode on the positive electrode with the external DC power supply, move the electrode potential to the positive direction (i.e. anode polarization), enter the passivation area of the E-pH diagram, so that the protected metal can be “passivated” and protected.
On the premise that the material meets the requirements, the corrosion of stainless steel pipeline in complex marine environment, such as high temperature, high humidity, high salt sea wind, salt fog, seawater splash and so on, is mainly caused by chemical corrosion and electrochemical corrosion caused by chloride ion. The corrosion of stainless steel pipe can be restrained or eliminated by changing the surface treatment, selecting materials with stronger corrosion resistance, coating on the surface of stainless steel pipe or electrochemical protection. It is hoped that the above methods will be of great help to similar projects in the future.
Source: China Stainless Steel Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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