1.What are the core risks?
Galvanic effect leads to accelerated loss of zinc coating
Performance: The zinc coating of the mixed joints will corrode 3-5 times faster than when used alone. In a short period of time (several months to 1 year), the zinc coating may peel off and expose the bottom, which will lead to rust of the base steel. If the environment contains chloride ions (such as coastal areas, industrial areas, and winter snow melting areas), the corrosion rate will be further accelerated.
Hidden dangers of "intergranular corrosion" of stainless steel
If the stainless steel is a non-stabilized steel, during high-temperature welding or in a 450-850℃ environment for a long time, carbon will combine with chromium to form chromium carbide, resulting in a decrease in the chromium content at the grain boundaries. At this time, the zinc ions produced by the corrosion of the galvanized layer may react with the oxide film on the surface of the stainless steel, destroying its passivation layer, accelerating the intergranular corrosion of the stainless steel, and causing a sharp drop in the mechanical properties of the material.
2.What are the risks of physical property mismatch?
Electrochemical potential difference at the contact part triggers "galvanic corrosion"
Even if no obvious electrolyte is formed, the close contact of the two metals may generate microcurrent due to the potential difference, which will cause partial dissolution of the zinc coating at the contact interface in the long term, forming crevice corrosion.
If there is vibration or stress, the friction at the contact part will damage the zinc coating and the stainless steel passivation film, further aggravating the corrosion.
Differences in thermal expansion coefficients lead to loose connections
The thermal expansion coefficients of galvanized steel and stainless steel are quite different. In an environment with drastic temperature changes, repeated thermal expansion and contraction can cause the connection to become loose, the gap to increase, and the intrusion of water vapor and pollutants to accelerate, forming a vicious cycle.
3.Do environmental factors exacerbate the risks?
Humid/high salt fog environment (such as coastal areas and chemical industry zones)
Water vapor will become an electrolyte, accelerating the galvanic cell reaction; chloride ions will penetrate the zinc coating and stainless steel passivation film, causing "pitting corrosion" or "crevice corrosion".
High temperature/high humidity environment (such as kitchen, bathroom)
High temperature will accelerate the chemical reaction speed, and high humidity will provide a continuous electrolyte environment, causing the corrosion products of the zinc coating (zinc oxide, zinc hydroxide) to loosen and fall off. At the same time, the stability of the passivation film of stainless steel decreases at high temperatures, making it more easily destroyed by zinc corrosion products.
4.What are the additional risks of improper connection methods?
Direct rigid connection
If the two are directly bolted or welded, the larger the contact area, the wider the "active area" of electrochemical corrosion. During welding, high temperature will destroy the galvanized layer, generate zinc vapor, and may also cause the chromium content in the stainless steel weld area to decrease, forming a "corrosion weak zone."
If there are gaps in the mixed joints, water vapor and pollutants will accumulate there, forming a "closed cell" with a corrosion rate much higher than that of the exposed surface (the pH value in the gap decreases, the chloride ions are concentrated, and the local corrosion is aggravated).
5.What is the ultimate impact of the risk?
Structural safety: After the galvanized layer fails, the base carbon steel will quickly rust, resulting in a decrease in component strength (such as bolt breakage and bracket deformation); if stainless steel undergoes intergranular corrosion, it will lose its toughness and may suddenly break under stress.
Surge in maintenance costs: Corrosion in mixed joints is often uneven and hidden, difficult to detect in the early stages, and requires frequent replacement of components or re-anticorrosion in the later stages. The maintenance cost may be 3-5 times that of using only one material.
Shortened lifespan: In harsh environments, the service life of mixed joint components may be only 1/3-1/2 of that of components made of a single material.