Discussion on corrosion data in saline solutions of sodium chloride in the corrosion data manual
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I already knew that austenitic stainless steel cannot resist corrosion from saltwater, but today I checked the corrosion data sheet and found that at concentrations below 30%, its performance is considered good (indicated by a checkmark). Experts, how should we interpret the data provided in this sheet? If it says the performance is good, then we can’t use it (we certainly wouldn’t choose 304 for use in salty water), so what’s the point of this sheet then? Please ask the experts for guidance, thank you!
In my opinion, it’s actually usable at 30%; in other words, if the medium can be kept below 30% at all times, there’s no problem using 304. But the reason it isn’t used in engineering applications is that it’s not possible to maintain a hydrochloric acid concentration below 30% in such settings; there is a risk that the concentration could increase due to evaporation. Since a project lasts for more than 15 years, no one wants to take that risk, and no one can guarantee that the concentration will remain constant. Therefore, naturally, no one dares to use it.
Chloride ions do not cause rapid overall corrosion of austenitic stainless steels, but there is a critical problem: pitting. 304 steel can develop perforations within just a few months, while 316 steel performs better. Steels with higher silicon content are more resistant to saltwater corrosion; however, the number 8 indicates a tendency toward pitting
The frozen saline delivery pipes and storage tanks can be made of non-metallic materials or composite materials, such as PP pipes and FRP tanks, which also reduces costs. Combined with a corrosion inhibitor, it can effectively extend the service life.
The information in corrosion data sheets is usually based on results obtained under standard test conditions, such as factors like temperature, concentration, stress state, and exposure time. When the manual indicates that austenitic stainless steel performs at a good level in sodium chloride solutions with concentrations below 30%, this means that its corrosion rate is acceptable under standard test conditions. However, in practical applications, environmental conditions can be more severe, such as high temperatures, alternating wet and dry environments, and mechanical stress, all of which can accelerate the corrosion process. Therefore, although the manual provides some guidance, when selecting materials, the actual usage environment and safety factors also need to be taken into consideration. 304 stainless steel is susceptible to pitting and stress corrosion in saline environments; therefore, more corrosion-resistant materials such as super duplex stainless steel or titanium alloys are often chosen in design. Therefore, the data in the manual is primarily intended as a reference; a comprehensive assessment taking into account actual circumstances is required for practical application. .