Corrosion is the type of loss that causes the most damage. The cause of corrosion is the chemical and electrochemical reaction of metallic materials with ambient media (operating media, water, steam, air, gas, coal gas, and the other similar substances). The definition of corrosion is: corrosion is the destructive invasion of metal caused by the chemical and electrochemical reaction between metal and its environment.
1. Effects of Corrosion
The important effects of corrosion are mainly in the following two aspects. Economic Impact: Material loss, inability to operate; Resource retention: Corrosion continues to reduce the world’s supply of metal resources. In Germany, 5 billion euros are invested every year in corrosion control or corrosion protection. In China, the detection of corrosion rate is also widely used in the oil and gas industry.
The main significance of the analysis of corrosion mechanism, corrosion rate and its causes is to estimate the service life of components, which is an important indicator for evaluating the uniform corrosion resistance of metals.
2. Factors Affecting Metal Corrosion
As the environment changes, various factors affecting corrosion become more and more complex, which will change the degree or type of corrosion and increase the severity of corrosion. There are many factors affecting metal corrosion. Metal corrosion is caused by various internal and external factors, mainly including the following:
- The chemical composition and structure of the metal material itself; the more active the metal, the easier it is to lose electrons and be corroded. If the conductive impurities in the metal are not as active as the metal, it is easy to form a primary battery and cause electrochemical corrosion of the metal;
- Ambient temperature and humidity, under normal circumstances, the increase of temperature will speed up the chemical reaction rate. Therefore, the effect of temperature on chemical corrosion is obvious.
- The composition and pH value of the solution in contact with the metal surface;
- Various environmental media in contact with the metal surface.
- Metal surface finish (battery corrosion due to oxygen concentration difference);
Metals are easily corroded when exposed to moist air, corrosive gases, and electrolyte solutions.
There are generally two ways to express the corrosion rate when the metal is uniformly corroded: one is expressed by the mass of the metal loss (or increase) per unit area per unit time, usually the unit is 2g/(mh); The other is to express the depth of metal corrosion per unit time, usually in mm/a.
3. How to measure the corrosion rate
At present, there are many methods for determining the corrosion rate, such as gravimetric method, volume method, polarization curve method (polarization resistance method) and so on. Gravimetric method is a classic method, which is suitable for laboratory and field coupons. It is one of the most reliable methods for determining the corrosion rate of metals. It can be used to detect the corrosion resistance of materials, select corrosive agents, and check the anti-corrosion effect when changing process conditions.
The gravimetric method measures the corrosion rate of metals according to the change in the mass of the specimen before and after corrosion, and is divided into two types: the weight loss method and the weight gain method. When the corrosion products on the metal surface are easy to remove and do not damage the metal body, the weight loss method is commonly used; when the corrosion products are completely and firmly attached to the surface of the specimen, the weight gain method is used.
The method of measuring the corrosion rate of metals in industrial production is to make small test pieces of metal materials, put them in a corrosive environment (such as chemical equipment, atmosphere, seawater, soil or experimental medium), and after a certain period of time, take them out and measure them. Changes in mass and size, and calculate the corrosion rate. Generally in the experiment, the metal is made into a test piece of a certain shape and size. After surface pretreatment, it is placed in a corrosive medium, taken out after a period of time, and the change in its quality and size is measured, and then its corrosion rate is calculated. But in the oil and gas industry, corrosion coupons are often used, we also call it the weight loss method.
Corrosion coupon is widely used as an important method to detect and monitor the corrosion of specific materials or media. The authenticity of its data will directly affect the relevant research results and the applicability of anti-corrosion measures, such as corrosion inhibition research and evaluation. As an important part of this method, the quality of corrosion coupon will have an important impact on the data.
The corrosion coupon is hung in the test vessel, on-site inspection heat exchanger or test pipeline, and is a standard metal test piece for corrosion measurement. It is used to detect or evaluate the corrosion condition of cooling water or other systems. Generally, the pre processed and weighed metal test pieces are placed in the test system for a period of time (such as 30-90 days), and the same group of test pieces can be taken out at different times, such as 30 days, 60 days, 90 days, etc. During long-term observation, put 12 or 24 test pieces at a time, take out 1 or 2 pieces every month, wash and weigh them, determine the weight loss degree of metal corrosion, the type of corrosion and corrosion depth, measure the corrosion rate respectively, and then the corrosion rate time curve is drawn.
In oilfield production and construction, the corrosion monitoring of pipelines is extremely important. In addition to the most commonly used corrosion monitoring methods, corrosion coupons, there are Corrosion probe, Linear Polarization Resistance (LPR), hydrogen probe, Inductive Impedance method and FSM full circumferential corrosion monitoring, etc.The purpose of corrosion monitoring is to master the corrosion process and understand the application and effect of corrosion control. Through corrosion monitoring, relevant information such as the correlation between the corrosion process and operating parameters can be obtained, the corrosion status and anti-corrosion measures can be evaluated, and the system can be identified. Corrosion factors, improve the corrosion control plan, and play a preventive role. The development direction of oilfield corrosion monitoring technology is that the advantages of various corrosion monitoring technologies complement each other and jointly promote the rapid development of corrosion protection research.