1.What is the corrosion load of the marine environment?
The marine environment is a "hell-level" scene for metal corrosion:
Salt spray/chloride ions: Chloride ions (Cl⁻) in seawater have strong penetrability and accelerate electrochemical corrosion.
High humidity: Continuous humid environment promotes oxidation reactions.
Microbial attachment (such as sulfate-reducing bacteria): accelerates local corrosion.
Splash zone + tidal range zone: Alternating dry and wet conditions cause the corrosion rate to be 3 to 10 times higher than that in the full immersion zone.
2.What are the shortcomings of galvanized steel in marine environments?
The zinc layer is rapidly consumed in salt spray (usually lifespan < 5 years), and once the zinc layer is exhausted, the carbon steel substrate will quickly rust and perforate.
3.What are the core advantages of 316 stainless steel in marine environments?
Chloride ion corrosion resistance
Molybdenum (Mo) support: 316 stainless steel contains 2~3% molybdenum, which significantly improves the ability to resist pitting and crevice corrosion.
Comparative experiment: In 3.5% salt spray, the corrosion rate of 316 is only 1/3 of 304 stainless steel and 1/20 of galvanized steel.
Passivation film self-repair: Even if the chromium oxide (Cr₂O₃) film is damaged, it can be quickly regenerated in oxygen-rich seawater.
Mechanical property stability
Strength retention rate: The tensile strength of 316 decreases by less than 5% after being immersed in seawater, while the weld area of galvanized steel is prone to rust embrittlement.
Stress corrosion cracking (SCC) resistance: 316L (low carbon type) has almost no SCC risk in seawater environment below 80℃.
4.What are the limitations of galvanized steel in marine engineering?
The zinc layer is consumed too quickly: Cl⁻ in seawater reacts with zinc to form soluble substances (such as ZnCl₂), which cannot form a protective layer.
Welding zone corrosion black hole: After the zinc layer in the heat-affected zone (HAZ) is burned, the rust rate is more than 5 times that of the parent material (see the figure below).
Galvanic corrosion risk: If in contact with stainless steel/copper alloy, galvanized steel as an anode will accelerate dissolution.
5.What are the engineering application scenarios of stainless steel?
Key structural parts: platform anchor chain, pressure pipe, valve (ASTM A182 F316 flange). Exposed parts: guardrail, gangway, fire protection system (316L impeller for seawater pump).
Special environment: submarine cable armor (316L wire anti-microbial corrosion), desalination equipment evaporator.