1.What causes microcracks?
Cause: After the plated part is taken out of the zinc pot, it cools down quickly. The thermal expansion coefficient of the zinc layer (especially the thicker zinc-iron alloy layer) is different from that of the steel substrate, and internal stress is generated during the cooling and shrinking process. Small cracks perpendicular to the surface are easily formed in corners, welds, local over-thickness or areas where the substrate stress is concentrated. Features: Usually fine, shallow, irregular mesh or linear cracks, mostly confined to the inside of the zinc layer. Commonly found in zinc-iron alloy layers.
2.What effect do shallow microcracks in unexposed steel substrate have on corrosion resistance?
The impact is small and usually acceptable:
The sacrificial protection of zinc is still working: Even if there are microcracks on the surface, as long as the zinc layer at the bottom of the crack still completely covers the steel matrix (that is, the crack does not penetrate), zinc can still protect the steel matrix below the crack through the "sacrificial anode" effect. The corrosion current will preferentially consume the zinc near the crack rather than developing deeper.
Corrosion products may block cracks: The alkaline corrosion products (such as zinc hydroxide and basic zinc carbonate) produced in the early stage of zinc corrosion have a certain volume, which may gradually fill and seal shallow microcracks and slow down further corrosion.
3.What effect do deep cracks that expose the steel substrate have on corrosion resistance?
The impact is serious, and the anti-corrosion performance is significantly reduced:
Direct exposure corrosion of the steel matrix: The crack reaches the steel matrix directly, forming a galvanic corrosion cell of "small anode (zinc)-large cathode (steel)". The exposed steel (cathode) is protected, but the large area of zinc (anode) around it will be consumed faster to protect the exposed point.
"Edge effect" amplifies corrosion: The zinc layer at the edge of the crack is accelerated to dissolve due to the concentration of current, causing the crack to widen and spread rapidly, destroying the surrounding protective layer.
High risk of pitting corrosion: The exposed point becomes the starting point of preferential corrosion (pitting), especially in a high chloride ion environment, and may quickly develop deep into the steel matrix.
Greatly shortened anti-corrosion life: Such defects are weak points in anti-corrosion. If not treated, the expected life of the overall component will be significantly reduced.
4.How to judge the harmfulness of microcracks?
Observe the depth and substrate exposure: Observe with the naked eye or a low-power magnifying glass: Is the metallic luster (grey-white) of the steel substrate visible at the bottom of the crack? If visible, the substrate is exposed.
Measure crack width and density: Cracks that are too wide (>0.5mm) or densely distributed are more risky. Location: Cracks located in high stress areas (such as near welds) and areas prone to water accumulation and corrosive media retention (such as grooves) are more harmful.
5.What are the preventive measures?
Optimize the galvanizing process: Control the zinc temperature, immersion time, cooling rate (such as slow cooling) to reduce thermal stress cracks.
Component design: Avoid sharp edges and corners, use rounded corners for transition; optimize welding process to reduce residual stress.
Careful handling and installation: Avoid mechanical damage, use appropriate lifting equipment and protective pads.
Choose high-performance coatings: In harsh environments or when the anti-corrosion requirements are extremely high, consider zinc-aluminum-magnesium coating (ZMA). It has stronger self-healing ability. Even if the coating has slight scratches or cracks, the corrosion products (containing magnesium and aluminum compounds) can seal the defects faster and provide better protection.