Which stainless steel is better for corrosion resistance, 304L or the 316L?
Due to the molybdenum, which is around 2-3% in 316L, it has a high corrosion resistance when exposed to seawater. It's also important to remember that isn't completely resistant to corrosion in every situation. In seawater it is susceptible to pitting and cracking corrosion, which limit its use in these environments. The corrosion resistance of 316L stainless in marine environments is affected by factors such as water quality, oxygen content, flow rate and temperature.
Water quality
Chloride levels in seawater vary depending on the geography and tides. The chloride level in brackish water is still higher than seawater. Crack corrosion can be a major concern. The risk of corrosion decreases when 316L is exposed intermittently to seawater in areas such as tidal zone. These cycles of exposure effectively clean stainless steel's surface. The evaporation from seawater splash zones can be more harmful to stainless steel 316L if there is a high concentration of chlorine in an environment with a lot of rain and humidity.
Avoiding unnecessary contact with seawater is essential. The cross section of 316L stainless seawater treatment pipe can pit in just a few months. To ensure the successful use of 316L in seawater-exposed areas, it is important to prevent cross pipe movement and maintain free drainage surfaces. After a hydrostatic seawater test on a stainless steel 316L pipeline, the system should be immediately flushed and drained to avoid local corrosion.
316l stainless steelCurrent velocity
It is preferable to have a higher flow rate than 1m/s. Slowly moving seawater may cause biofouling and crevice corrosion. Stastified seawater should be avoided. The risk of corrosion is reduced when the flow rate increases. This makes 316L stainless suitable for applications that involve seawater, such as pumps or other equipment used to treat seawater.
Water temperature
As the temperature rises, cracking corrosion intensifies. It is recommended that 316L do not come in contact with seawater. In the Nordic region, water temperatures can exceed what 316L can tolerate at room temperature. Temperatures higher than these can cause cracking and pitting.
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Oxygen content in water
The passive nature of 316L stainless is dependent on oxygen. Oxygenated seawater, however, is much more aggressive than seawater that has been degassed. The corrosion of stainless steel 316L is reduced by seawater that contains very little oxygen, for example, at depths of 200 meters.
Cathodic protection
Noble secondary metals like aluminum and carbon steel are used for electrical contact.Direct contact can increase the corrosion resistance in 316L, but it could also sacrifice other metals. This combination can weaken the durability of the products.
Processing cracks
When 316L is used in saltwater it can experience pitting corrosion, which has a direct link to cracking. This may be due to factors like sharp edges, grooves and flange seals.In order to use 316L in seawater with low chlorine, the quality of the surface welds as well as the smoothness and cleanliness of the stainless surface is important. The corrosion resistance of stainless steel 316L is influenced by the cleanliness, smoothness and flatness of surface welds.
The excellent corrosion resistance offered by 316L stainless is unquestionable. Due to the limitations of corrosion resistance and applications for 304 stainless and 304L, 316L is in high demand.
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