Now in the marine environment has a large number of stainless steel pipe materials and products, in order to adapt to all kinds of corrosion damage of the marine environment, we have developed a special type of stainless steel pipe, here to introduce in the marine environment commonly used type of stainless steel pipe products.
303 stainless steel pipe is a kind of containing sulfur and selenium cutting stainless steel pipe, mainly used to meet the cutting and roughness comparison high demand scenarios. As one of the most widely used stainless steel pipe 304 stainless steel pipe, corrosion resistance, heat resistance, low temperature strength and mechanical properties, punching, bending and other thermal processing performance is very good, and does not produce heat treatment hardening. As for the 316 stainless steel pipe, because of its chemical components added molybdenum elements, so its corrosion resistance, corrosion resistance and high temperature strength is very good, can be used in a relatively harsh environment, it has good performance of work hardening, no magnetism, very suitable for use in the marine environment. The other 321 stainless steel pipe is based on 304 stainless steel pipe on adding titanium to prevent the occurrence of intergranular corrosion, suitable for use in environmental temperature of 430 DEG -900 DEG is used, but the disadvantage is higher cost, and the processing performance is less than that of 304 stainless steel pipe.
So as you can see, is widely used in the marine environment for more than 316 stainless steel pipe, 316 stainless steel pipe in contact with seawater but the corrosion resistance is limited, can not resist corrosion under all conditions, because is more susceptible to localized corrosion, cracks and pitting corrosion mainly. This limits the range of application of 316 in contact with seawater. 304 stainless steel pipe, especially the easy cutting 303 stainless steel pipe, is not suitable for working in seawater. The sulfide inclusion point of the 303 surface is the place where pitting occurs preferentially.
The factors that determine the corrosion resistance of 316 steel in sea water include water quality, flow rate, temperature, oxygen content and cathodic protection.
The chloride content in water will be different from the influence of region and tide. Even in slightly salty water, the content of chloride is also higher than that in the water where corrosion is the threat of corrosion. Intermittent exposure in seawater, such as tidal zone, corrosion risk will be smaller, this is probably because of the steel surface by the change of water level of effective erosion. However, if the concentration of chloride in the humid and rainy environment, in the splash zone of water evaporation will increase the risk of corrosion. In addition, slow moving water contributes to biological contamination, and then leads to protection or crack corrosion. It’s better not to use 316 stainless steel pipes in stagnant seawater. Increasing the flow rate will reduce the risk of corrosion, so that 316 stainless steel pipes can be used in seawater.
Crack corrosion will increase with temperature, so it is not recommended to contact hot seawater. Nordic waters reach a maximum temperature of 316 steel at room temperature, even though other conditions are good. Stress corrosion cracking is usually not considered at 316 service temperature (in any case, higher temperature may lead to crack corrosion and pitting corrosion).
Oxygen content in water (except gas)
The stainless steel pipe depends on oxygen to maintain its passivation state, however, the oxygenated seawater is more aggressive than degassing. It has been found that the water with very low oxygen content, such as in the sea about 200 meters deep, has very little erosion. This reduces the pitting rate.
Cathodic protection is used in electricity to obtain contact with inert weak metals, including carbon steel and aluminum. Direct contact with these metals, on the basis of sacrificing other metals, helps to improve the corrosion resistance of 316 stainless steel pipes. Although the stainless steel pipe is beneficial, it should also be considered that the overall durability of the products involved in this combination may be weakened.
The crack and its closely related pitting mechanism are localized corrosion forms, which are usually the cause of damage to the 316 steel working in seawater. They can happen in the following process.
Mechanical fastening system
Intergranular corrosion can be detected in 316 stainless steel pipes exposed to seawater after laboratory sensitization (heat treatment). The application of low carbon 316L such as 1.4404 or 1.4432 should avoid the additional corrosion risk in the welded structure. Smooth surface welding quality and finishing are the key factors for the successful application of 316 stainless steel pipes in seawater, such as the actual chlorine content and other factors. Smooth, clean, leveling and finishing welds contribute to the corrosion resistance of steel.
Source: China Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)