1.Where is the corrosion mechanism of acid rain on zinc layer manifested?
The main components of acid rain are sulfuric acid and nitric acid, and may also contain a small amount of hydrochloric acid.
Chemical dissolution reaction
Zinc is an active metal and reacts directly with H⁺ in an acidic environment.
Accelerated electrochemical corrosion
The water film formed by acid rain is an electrolyte, and the zinc layer undergoes a "corrosion battery" reaction.
Influence of corrosion products
The corrosion products of zinc are mainly Zn (OH)₂, ZnSO₄・7H₂O, etc. These products have poor coverage on the surface of the zinc layer: if acid rain erodes frequently, the corrosion products are easily eroded and cannot form a protective oxide film, and the zinc layer continues to be exposed and corroded;
If the environmental humidity is high and there is little erosion, the products may accumulate locally, but due to the loose texture, they will continue to corrode the underlying zinc layer through the gaps.
2.What is the effect of acid rain pH on the zinc layer loss rate?
pH value is the core indicator that determines the strength of acidity, and its influence on zinc layer loss is nonlinear: When pH > 6.0 (weakly acidic or neutral): zinc layer corrodes slowly, and the loss rate is usually < 0.5 μm / year (mainly relying on the weakly acidic water film formed by a small amount of CO₂ in the atmosphere);
When pH = 4.0-6.0 (moderate acid rain): H⁺ concentration increases significantly, and the loss rate increases to 2-5 μm / year;
When pH < 4.0 (strong acid rain, pH can be as low as 2.0-3.0): zinc layer reacts violently, and the loss rate can reach 10-20 μm / year (such as heavy industrial areas and around coal-fired power plants).
3.What is the impact of acid rain composition and concentration on the zinc layer loss rate?
Sulfate and nitrate: When the concentration of SO₄²⁻ in rainwater is greater than 100 mg/L (up to 200-500 mg/L in heavily polluted areas), it will combine with zinc ions to form easily soluble zinc sulfate, accelerating the dissolution of the zinc layer;
Chloride ions (Cl⁻): If acid rain is accompanied by sea salt particles (such as coastal areas), when the concentration of Cl⁻ is greater than 50 mg/L, it will destroy the local passivation of the zinc surface and induce pitting corrosion, resulting in a local loss rate 3-5 times faster than uniform corrosion.
4.What effect does the exposure environment have on the rate of zinc layer loss?
Humidity and rainfall frequency: In areas with high humidity and frequent rainfall, the zinc layer is in a wet state for a long time, the corrosion reaction continues, and the loss rate is 2-3 times higher than that in dry areas;
Temperature: For every 10℃ increase in temperature, the corrosion reaction rate increases by about 1 time, so the zinc layer in summer or tropical areas loses faster than in winter/frigid areas;
Sunlight and ultraviolet rays: The zinc layer exposed outdoors is exposed to ultraviolet rays, which may accelerate the aging and peeling of surface corrosion products, indirectly accelerating the loss.
5.What are the measures to slow down the loss of zinc layer in acid rain environment?
Increase the thickness of the zinc layer: In areas with strong acid rain, use a thick zinc coating to extend the protective life;
Strengthen surface treatment: Passivate the zinc layer or apply an organic coating to form a double protection, which can reduce the loss rate by 60%-80%;
Regular maintenance: In areas with moderate acid rain, clean the corrosion products on the zinc layer surface and reapply protective agent every 1-2 years to maintain the protective effect;
Environmental isolation: For indoor equipment, reduce humidity through ventilation, or use a sealed cover to reduce direct contact with acid rain.