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## I. Electrochemical Corrosion Principle of Zinc Coatings
1. Microscopic Corrosion Cause: Defects exist in the purity and uniformity of zinc coatings, forming countless micro galvanic cells in corrosive media and triggering localized electrochemical corrosion. Zinc dissolves and loses electrons at the anode region, while oxidants in the media gain electrons at the cathode region; directional migration of electrons and ions forms a complete conductive loop.
2. Sacrificial Anode Protection Mechanism: In 3% NaCl solution, the standard potential of Zn/Zn²⁺ is far lower than that of Fe/Fe²⁺. When the coating is damaged, zinc preferentially dissolves as the anode, and the steel substrate acts as the cathode and is protected.
3. E-pH Diagram of Zinc (Potential-pH Diagram)
- Zones: Corrosion Zone A, Passivation Zone B, Corrosion Zone C, Immunity Zone D. A pH value maintained between 8 and 12 facilitates the formation of a passive film to achieve passivation protection.
- Application: A core tool for aqueous corrosion research used to predict corrosion tendency, corrosion products and applicable service environments.
- Limitations: Only applicable to equilibrium states at a normal temperature of 25°C; unable to calculate corrosion rates, and changes in temperature or alloy elements will shift the range of each zone.
4. Side Reactions in Aqueous Corrosion: Oxygen evolution occurs when the potential exceeds line m, and hydrogen evolution takes place when the potential falls below line n. The corrosion of zinc is accompanied by cathodic reduction reactions of hydrogen evolution or oxygen reduction.
## II. Dual Protective Effects of Zinc Coatings on Steel
1. Barrier Protection
An intact zinc coating forms a dense physical barrier that isolates the steel substrate from corrosive media and inhibits the occurrence of electrochemical corrosion.
2. Cathodic (Sacrificial Anode) Protection
Zinc has a lower potential than iron. When a galvanic cell is formed, zinc continuously dissolves sacrificially to protect steel. The protective performance is retained even if the coating is cracked or locally discontinuous. Its service cycle is divided into three stages:
(1) Initial Stage: A corrosion product film forms on the surface to provide long-distance electrochemical protection with an ideal protection distance of 1–2 mm. In high-humidity environments, the corrosion products become soluble, drastically accelerating zinc corrosion.
(2) Middle Stage: The longest service phase with corrosion behavior similar to pure zinc. The passive film is continuously generated and consumed, leading to slow thinning of the zinc coating.
(3) Final Stage: The zinc coating is exhausted and the steel substrate is exposed, triggering long-distance sacrificial anode protection again.
3. Shift in Dominant Protection Mechanisms
Sacrificial electrochemical protection dominates in the initial service period, while passive film formation and water vapor permeation resistance become the primary protective mechanisms in the middle and late stages.
4. Comparison of Different Damaged Coatings (Figure 1–4)
- Cracked Galvanized Coating: Zinc corrodes preferentially and the steel substrate remains protected.
- Cracked Paint Coating: Exposed steel rusts directly, and rust spreads continuously to damage the entire paint film.
- Cracked Copper Plating: Copper has a higher potential, turning iron into the anode and accelerating its corrosion with no protective effect.
## III. Corrosion Resistance of Zinc Coatings in Various Environments
1. Atmospheric Environments (Key influencing factors: relative humidity, SO₂ concentration, coating thickness)
Service life increases with coating thickness. The ranking of atmospheric corrosion severity is: rural area < coastal area < marine atmosphere < suburban industrial area < industrial area.
(1) Clean Atmosphere: A dense basic zinc carbonate passive film forms, delivering excellent corrosion resistance.
(2) SO₂-containing Industrial Atmosphere: SO₂ is the most aggressive atmospheric pollutant. High humidity acidifies the surface water film and destroys the passive film. There exists a critical SO₂ concentration, beyond which the corrosion acceleration rate slows down.
(3) CO₂-containing Atmosphere: Protective basic zinc carbonate forms, but acidification of the surface water film weakens the stability of the passive film.
(4) Chemical Atmosphere with H₂S/NH₃/HCl: All three substances accelerate corrosion. H₂S acidifies the water film, NH₃ generates soluble zinc complexes, and HCl causes strong acid corrosion.
(5) NO₂-containing Atmosphere: Acidified deposited particles accelerate zinc coating corrosion.
(6) Marine Atmosphere: Chloride ions reduce critical humidity, improve water film conductivity and dissolve passive films, significantly accelerating corrosion.
2. Aqueous Environments
Corrosion rate is jointly determined by pH value, water temperature and water impurities:
(1) Effect of pH: Optimal corrosion resistance is achieved at a normal temperature pH range of 6–12; corrosion intensifies sharply under strong acid or strong alkaline conditions. When pH > 12.5, zinc reacts violently with alkalis and releases hydrogen gas.
(2) Effect of Temperature: Corrosion rate rises steeply from 20°C to 65°C and peaks at 65°C, followed by a decline with further temperature increase.
(3) Water Impurities
- Carbonic Acid and Nitric Acid: Zinc acts as the cathode without corrosion.
- Chlorides and Sulfates: Zinc turns into the anode and corrodes rapidly; polarity reversal of zinc-iron occurs when air is bubbled through hot water at 60°C.
(4) Seawater: Magnesium salts inhibit corrosion. A 25 μm-thick zinc coating has a service life exceeding one year, superior to salt water of the same concentration.
3. Soil and Building Material Environments
(1) Soil: Corrosion is comprehensively affected by water content, dissolved oxygen, salt content and pH. Oxidized inorganic soils support a coating service life of more than 10 years, while reducing soils drastically shorten service life.
(2) Corrosion Rate Ranking for Contact with Building Materials: fiberboard/asbestos cement < red brick < slag wool < cement < gypsum. H₂S-containing underground water in oilfields corrodes zinc coatings rapidly. Annealing at 500–550°C to form an Fe-Zn alloy layer can greatly extend anti-corrosion service life.
4. Contact with Dissimilar Metals (Galvanic Corrosion)
Contact between dissimilar metals forms galvanic cells, and corrosion severity depends on the potential difference between metals:
(1) Copper/Brass: Potentials are much higher than zinc, causing rapid zinc corrosion under humid conditions. Insulating gaskets and water flow isolation are required to cut off galvanic conduction.
(2) Aluminum/Stainless Steel: No obvious corrosion occurs under normal dry conditions; coating isolation is needed for high-humidity service environments.
(3) Weathering Steel: Zinc continuously dissolves as a sacrificial anode until a stable rust layer forms on the steel surface. Sufficient zinc coating thickness is required for long-term protection.
# Core Overall Conclusions
1. Zinc coatings protect steel through the dual mechanisms of physical barrier isolation and sacrificial anode protection, and retain protective performance after coating damage, making them a mainstream long-lasting metal anti-corrosion coating.
2. Environmental humidity, acidic pollutants, chloride ions, temperature and pH value are core factors governing zinc coating corrosion rates. Industrial, marine, strong acid and strong alkaline environments drastically shorten service life.
3. Direct contact between zinc and high-potential metals (copper, brass) accelerates zinc consumption, so insulating isolation must be implemented in engineering design.
4. E-pH diagrams can qualitatively determine zinc corrosion zones but cannot quantitatively calculate corrosion rates, presenting inherent limitations in practical application.