1.What is a highly corrosive environment?
Chemical media: strong acids (such as sulfuric acid, hydrochloric acid), strong alkalis (such as sodium hydroxide), high-concentration salt solutions (such as seawater, industrial waste liquid), oxidizing media (such as sodium hypochlorite).
Gas environment: industrial waste gas containing SO₂, H₂S, Cl₂, or high salt fog environment in coastal areas.
Complex factors: high temperature + high humidity + coexistence of corrosive media (such as chemical reactor pipelines, waste incineration plant flue gas pipelines).
2.What is the corrosion mechanism of hot-dip galvanized elbows?
The protection principle of hot-dip galvanized layer (zinc content ≥ 98%) is sacrificial anode protection: zinc is more active than iron and reacts with corrosive media first, thereby protecting the steel substrate. However, in a highly corrosive environment, the zinc layer will accelerate the loss due to the following reasons:
Electrochemical corrosion: When there is an electrolyte (such as water, salt solution) in the medium, the zinc layer and the substrate form a galvanic cell, and zinc continues to dissolve as an anode.
Chemical dissolution: Strong acid/strong alkali reacts directly with zinc (such as hydrochloric acid reacts with zinc to produce zinc chloride and hydrogen), causing the coating to thin rapidly.
Intergranular corrosion: If the galvanizing process is improper (such as impurities or stress concentration in the coating), the corrosive medium may penetrate along the grain boundary of the zinc layer, causing local peeling.
3.What are the typical failure modes of hot-dip galvanized elbows?
Uniform corrosion: A large area of "white rust" (basic zinc carbonate) or "red rust" (iron rust) appears on the surface of the zinc layer, and the thickness of the coating decreases year by year, eventually exposing the substrate.
Pitting perforation: Under the action of chloride ions (such as seawater and industrial wastewater), tiny holes are formed on the surface of the zinc layer, which then extend to the substrate, causing pipeline leakage.
Stress corrosion cracking: In a high temperature or high pressure environment, the corrosive medium and mechanical stress work together to cause cracks in the elbow weld or corner.
4.How do hot-dip galvanized elbows perform in strong acid/alkaline environments?
The life of hot-dip galvanized layer in dilute acid at room temperature (such as 5% hydrochloric acid) is about 5 to 8 years, but when the concentration exceeds 20%, the zinc layer may be completely dissolved within 1 to 2 years.
Strong alkali (pH>12) will react with zinc to form soluble zincate, causing the coating to gradually soften and easily fall off under high pressure flushing.
Failure characteristics: The inner wall coating of the elbow is honeycomb-like corroded, and the metallic base material is partially exposed.
5.How do hot-dip galvanized elbows perform in high-salt/seawater environments?
Chloride ions (Cl⁻) penetrate the zinc oxide film to form an electrochemical corrosion cell, which accelerates the loss of zinc. In the salt spray test (GB/T 10125), the hot-dip galvanized layer will develop red rust (ordinary hot-dip galvanizing) in about 500~1000 hours under the spray of 5% NaCl solution, while the thickened coating (≥85μm) can be extended to more than 1500 hours.
Failure characteristics: Dense white salt crystals precipitate on the outer wall of the elbow, and the inner wall is partially thinned due to water erosion.