Five types of corrosive environments for coating anti-corrosion

Atmospheric corrosion

Atmospheric corrosion is a process that occurs within a wet film on the surface of steel structures. The wet film layer may be too thin to be seen with the naked eye.

The following factors can lead to an increase in the corrosion rate:

The increase of relative humidity

Condensation occurs (when the surface temperature is at or below the dew point);

The total amount of air pollutants has risen (corrosive pollutants can react with steel and may form deposits on the surface).

Experience shows that severe corrosion mostly occurs when the relative humidity is greater than 80% and the temperature is above 0℃. However, if pollutants and hygroscopic salts are present, corrosion will also occur at lower humidity. The atmospheric humidity and air temperature in a specific area on the Earth depend on the local climatic conditions. The positions of each component of a steel structure can also affect corrosion. Therefore, steel structures exposed to the open air will suffer from corrosion under the influence of climatic parameters such as rain, sunlight, gases or suspended pollutants. While the impact of indoor air pollutants is reduced, there may still be local high corrosion caused by insufficient ventilation, high humidity or condensation. For the judgment and assessment of corrosion stress, the correct identification of the local environment and the micro-environment is very important. Some decisive micro-environments such as under Bridges (especially on water), the roofs of indoor swimming pools, the sunny and shady sides of buildings.

Corrosion in water and soil

Special attention should be paid when a part of the steel structure is immersed in water or buried in the soil. In such cases, corrosion usually concentrates in a small area where the corrosion rate is very high. Exposure tests are not recommended to assess the corrosiveness of water and soil environments. However, different situations of water immersion and soil burial can still be described.

Structures immersed in water

The types of water, including fresh water, salt water or seawater, have a serious impact on the corrosion of steel. Corrosiveness is also affected by the oxygen content in water, the type and quantity of dissolved substances, and water temperature. The growth of animals and vegetables can also accelerate corrosion. The water-impregnated environment can be divided into the following three types of areas:

Underwater area: A place permanently submerged in water;

Intermediate area (fluctuating water level) : The area where water level fluctuates due to natural or human factors and is subject to the intermittent combined action of water and atmosphere, resulting in aggravated corrosion.

Splash zone: The area splashed by waves and water mist, which can cause abnormally high corrosion stress, especially in salt water.

Structures buried in the soil

The corrosiveness of soil is related to the mineral content and physical properties, organic matter content, water content and oxygen content in the soil. The corrosiveness of soil is greatly influenced by aeration conditions. Changes in oxygen content can lead to corrosion and the formation of a galvanic cell effect. In most steel structures such as pipelines, tunnels, storage tank installations, etc., which are located in different types of soil, soils with different oxygen contents, soils with different groundwater levels, etc., local corrosion (pitting corrosion) begins to intensify due to the corrosion cell effect.

Special circumstances

When choosing a coating protection system, both the special corrosion stress that the steel structure is subjected to and the special location of the steel structure should be taken into consideration, such as the erosion by corrosive media like acids, alkalis and salts.