The problem of metal corrosion pervades many fields of the national economy, such as oil and gas, petrochemical, transportation, machinery manufacturing, etc. Corrosion has brought many losses and harms to social production.
1. Economic loss
The economic losses caused by corrosion are staggering. According to domestic rough statistics, the economic loss caused by corrosion has reached 280 billion yuan, accounting for about 4% of the gross national product (GNP). Looking at the global oil and gas production industry, according to the estimate of the National Association of Corrosion Engineers (NACE), the total global annual corrosion cost is 1.3 billion US dollars, about 8.6 billion RMB.
2. Environmental pollution
The waste of resources and environmental pollution caused by corrosion are extremely serious. Corrosion can cause a large amount of rust, causing premature failure of metal materials, and also causing a lot of waste of resources and energy. Corrosion products and dirt will reduce the heat transfer efficiency of boilers, heat exchangers and other equipment, wasting a lot of tons of standard coal. Harmful gases such as SO2 produced by burning these coals have aggravated environmental pollution.
3. Security risks
In 2011, a petrochemical plant located in the deethanizer top reflux tank of the gas fractionation unit suddenly exploded, resulting in 4 deaths, 1 serious injury, 6 minor injuries, and a direct economic loss of 8.69 million yuan. According to the accident investigation, the cause of the accident was hydrogen sulfide stress corrosion, which caused the rupture of the reflow tank and caused an explosion.
What is Metal Corrosion?
Metal materials are damaged by the action of the surrounding medium, which is called metal corrosion (Metallic Corrosion).
What are the types of metal corrosion?
Due to the complexity of corrosion phenomena and mechanisms, the situation and characteristics of metal corrosion vary greatly due to the differences in materials and environmental factors, that is, the stress state. Therefore, there are many classification standards and methods, but the common corrosion classification methods include classification by corrosion mechanism and classification by corrosion form.
How to classify according to corrosion mechanism?
According to the corrosion mechanism, it can be divided into chemical corrosion and electrochemical corrosion:
Chemical corrosion refers to metal corrosion caused by the direct chemical reaction between metal and non-electrolyte.
Electrochemical corrosion is the destruction caused by the electrochemical interaction between metal and electrolyte solution.
How to classify according to corrosion form?
According to the classification of corrosion form, it can be divided into general corrosion and localized corrosion:
General corrosion occurs over the entire metal surface and it may be uniform or uneven.
Localized corrosion is concentrated in some areas of the metal, while other parts have little or no corrosion. There are many types of local corrosion, generally divided into the following categories:
Galvanic corrosion: In an electrolyte solution, when dissimilar metals are in contact, the metal with a higher potential promotes the metal with a lower potential to accelerate corrosion;
Pitting corrosion: Corrosion damage is mainly concentrated on some active points, the diameter of the corrosion hole is equal to or smaller than the depth of the corrosion hole, and in severe cases, it can lead to equipment perforation;
Crevice corrosion: corrosion that occurs in crevices such as riveting, threaded connections, welded joints, and gaskets;
Intergranular corrosion: Corrosion proceeds along the intergranular, the bonding force between metal atoms is lost, and the mechanical strength of the metal decreases sharply. There is often no obvious change in the appearance of the metal before damage;
Stress corrosion: A cracking failure that occurs and expands at a significant rate under the combined action of tensile stress and corrosive medium; the common types are sulfide stress corrosion cracking and hydrogen sulfide stress corrosion cracking.
Corrosion fatigue: Corrosion of metals under the action of corrosive media and alternating stress.
How to reduce the impact of corrosion on the application in the actual application process?
1. Determine the type of corrosion
In the field application, the type of corrosion is judged according to the type of medium, pressure, temperature, environmental conditions and corrosion location, and the appropriate anti-corrosion method is selected. For example, in the oil and gas industry, the instrument pipe valve parts and the main process pipeline use products of different materials, which may lead to galvanic corrosion.
2. Select the appropriate corrosion-resistant material.
When selecting materials, it is necessary to consider the internal corrosive medium type, pressure and external environment. If the environment exceeds a certain temperature limit, it is not recommended to use 316 stainless steel at sea.
3. Reduce the position where crevice corrosion may occur.
Reduce contact with incompatible metals. Offshore, the risk of crevice corrosion is high when metal tubing and metal tube supports are used together. It is recommended that when selecting a tube support material, choose a tube support with a gasket.
Corrosion is a costly problem. If the type of corrosion and the location of the corrosion can be quickly identified, the potential risk can be greatly reduced. Technicians with professional knowledge can often predict where corrosion may occur, and select appropriate metal materials in advance to avoid corrosion. This will greatly improve the performance, integrity and security of production systems.
