1.What are the effects of missing plating/exposed iron?
In areas where zinc is not present, the substrate (steel plate) is not covered by the zinc layer and is directly exposed to the corrosive environment. Because iron has a higher electrode potential than zinc (making it more susceptible to corrosion), the exposed iron quickly develops "iron rust." Even if there is a surrounding zinc layer, the exposed iron spot becomes a "corrosion core," quickly developing red rust. This rust then gradually spreads beneath the surrounding zinc layer ("undermined corrosion"), resulting in a larger area of corrosion failure.
For example, in outdoor galvanized steel, visible red rust will appear within one to two weeks of the rainy season. Within three months, the rust can spread to a radius of 5 to 10 mm around the missed area.
2.What impact do cracking/peeling/blistering have on corrosion resistance?
Cracks or peeling in the zinc layer (separation of the zinc layer from the substrate) prevent the zinc layer from protecting the substrate through the "sacrificial anode effect." Cracks break the zinc layer, allowing moisture and salt to penetrate the gap between the zinc layer and the substrate through the cracks, creating a "localized electrolyte environment." This weakens the electrical connection between the zinc layer and the substrate, making it unable to effectively "sacrifice" itself to protect the iron. Instead, a dual reaction of "zinc corrosion + substrate rust" occurs within the gap: corrosion accelerates the zinc layer's shedding due to the cracks, while the substrate rapidly rusts due to the loss of the zinc layer's protective adhesion.
If blistering (gas or liquid accumulation under the zinc layer) occurs, a "closed corrosion environment" forms within the blister, where corrosion rates are faster than in an open environment. Typically, the zinc layer will completely detach within the blister within 3-6 months, leaving the substrate with extensive rust.
3.What impact do severe inclusions/nodules have on corrosion resistance?
Surface inclusions of zinc slag, impurities (such as uncleaned iron filings), or raised zinc nodules can form an "electrochemical microbattery" with the zinc layer. Impurities (such as iron filings) have a higher electrode potential than zinc, acting as the "cathode." The zinc layer, acting as the "anode," accelerates dissolution (corrosion) around the impurities. Furthermore, raised zinc nodules are prone to friction and fall off. Furthermore, the nodules are not tightly bonded to the substrate, resulting in a thinner zinc layer around them. These exposed areas become new corrosion sites.
For example, when iron filings are present on the surface of a galvanized sheet, the zinc layer within 1-2 mm of the surrounding area will corrode preferentially (forming white rust) within one month. Within six months, the zinc layer will completely disappear, and the exposed substrate will begin to rust.
4.How do deep scratches/dents affect corrosion resistance?
If the depth of the scratch/indentation exceeds the thickness of the zinc layer (exposed iron), it is equivalent to "local non-plating", and the corrosion process is the same as non-plating; if no iron is exposed but the zinc layer becomes thinner, the zinc layer in that area will be "consumed prematurely" - the anti-corrosion life of the galvanized sheet is positively correlated with the thickness of the zinc layer, and the thin zinc layer area will be corroded before other areas (for example, for a galvanized sheet with a standard zinc layer thickness of 80g/㎡, the zinc layer only remains at 30g/㎡ at the scratch. The sheet that can originally be corrosion-resistant for 5 years will have base material rust at the scratched area within 2-3 years).
5.What effect does uneven zinc layer thickness have on corrosion resistance?
If the zinc coating thickness on a galvanized sheet varies significantly (e.g., 100g/m² thick in one area and only 50g/m² in another), the thinner areas will corrode first (zinc dissolves to form zinc ions, acting as a sacrificial anode). Once the thinner areas are completely consumed, even if the zinc coating in other areas remains intact, the base material in these areas will begin to rust, rendering the entire sheet useless due to "localized failure."
For example, if the zinc coating on the edges of a galvanized sheet used for shipbuilding is only 50% of the thickness of the center (thin edges, thick center), in a high-humidity, high-salt marine environment, the edges will rust within 3-6 months, while the center will remain intact. However, the rust from the edges will gradually spread to the center, causing the entire sheet to be scrapped prematurely.