Stainless Steel----316, 304 and 303 stainless steel using seawater selection
316 steel is widely used in the marine environment , but they are in contact with sea water corrosion limited time , corrosion can not in all states. They are vulnerable to localized corrosion , mainly crevice corrosion and pitting . This limits their application in contact with the seawater environment. 304 steel, especially steel cutting 303 are unfit for work in seawater. 303 points sulfide surface pitting priority areas .
Determinants of 316 steel in seawater corrosion resistance
Many workers engaged in this field of research in the literature have reported 316 steel corrosion resistance and determine the applicability of factors. These factors together , including: water quality, flow rate, temperature , oxygen content, cathodic protection .
Water : The affected area and tides , chloride content is different. Even in brackish water , chloride content is also higher than the content of corrosion crack corrosion constitute a threat in the water. Intermittent exposure to sea water , such as in tidal areas, the risk of corrosion will be smaller , it may be effective scouring effect occurs because the steel surface by the water level changes , however , if the chloride and concentrated in wet rainy environment, the case of the splash zone water evaporation will increase the risk of corrosion , unnecessary, and do not make contact with seawater steel is very important. Treated seawater pipe cross -section 316 only in the short term may be pitting damage. Free-draining surface and avoid cross tube 316 mobile is very important for the success of the steel used in the field of contact with seawater . If the pipe system using seawater hydrostatic test, then the test must be immediately drained and rinsed , otherwise it will cause corrosion on the 316 system .
Flow rate : the higher the flow rate is desirable ( typically greater than 1m / s). Slow-moving water helps biological contamination , which then leads to cracking or corrosion protection . Be sure to avoid stagnant sea state .
Increase the flow rate will reduce the risk of corrosion , therefore , in dealing with water, such as pumps and other fields , can use 316 steel.
Water temperature: crevice corrosion increases with temperature , so the water is not recommended steel thermal contact . Nordic waters at room temperature can be used on about 316 steel reached maximum temperature , even if the other conditions are favorable . Stress corrosion cracking is usually not considered in the 316 temperature ( in any case, higher temperatures may lead to crevice corrosion and pitting ) .
Oxygen content ( except gas ) Water: stainless steel passivation rely on oxygen to maintain their status , however , is stronger than oxygenated seawater erosion degassing . It has been found very low levels of oxygen in the water , such as in about 200 meters of the sea , a small erosion . This reduces the pitting corrosion rate.
Cathodic Protection : Cathodic protection for electricity , and by inert weaker metals including steel and aluminum contacts available. Direct contact with the metal , at the expense of other metal base 316 helps to improve the corrosion resistance of stainless steel . Although stainless beneficial , but should also consider involving this combination of products integrated durability may be weakened.
Processing crack ( surface smooth and clean after processing ) : cracking and pitting closely related forms of localized corrosion mechanism is usually the cause of damage when 316 steel work in seawater . They can occur in the following procedure .
Mechanical Fastening Systems: intergranular corrosion in the laboratory sensitization ( heat treatment ) after exposure in seawater 316 steel detected. Such as the application of low-carbon 316L 1.4404 or 1.4432 should avoid welded structures such additional risk of corrosion . Weld quality and smooth surface finishing of steel in seawater is important in the successful application of reason steel 316 , it may be other factors such as the effective chlorine content is more important. Smooth, clean and flat welds and finishing all contribute to the corrosion resistance of the steel.
Key Words: Stainless steel, acero inoxidable