Detailed explanation of different defects of hot-dip galvanized steel plates

Causes of the "chicken claw pattern" defect on the surface of hot-dip galvanized steel plates
Defect Description: A macroscopic pattern very similar to the "chicken claw print" appears on the surface of the hot-dip galvanized steel strip. Because its morphology is similar to chicken claws, it is usually called a "chicken claw pattern" on site. It looks very rough. If you look closely, there are many dense black spots in the local area. This defect can appear on the upper and lower surfaces of the strip and has a slight feel. This type of defect is very common on the surface of strip steel with strength levels of DP780 and above, and is very common in galvanizing lines without pre-oxidation-reduction process sections, while DP590 and below levels are relatively rare.

Surface transition layer morphology of the missed plating area: Dezincification treatment is performed on the defective area to remove the surface zinc layer. The key point of this test is to grasp the corrosion scale and retain the transition layer structure as much as possible. If the corrosion technique and experience are not well mastered, it is easy to completely corrode the transition layer. Generally, hydrochloric acid and distilled water are mixed and diluted in a 1:1 ratio, and then the galvanized sheet is clamped with tweezers and placed in dilute hydrochloric acid at an angle of 45 degrees. Half of the sample is in the solution and half is in the air. The advantage of this corrosion is that a gradient corrosion structure can be obtained at the junction of dilute hydrochloric acid and air. This transition area can obtain a gradient corrosion area, and a relatively ideal transition layer structure can be found from this area.

Microstructure characteristics of the transition layer in the defective part: there will be a clear dividing line, the left side is the leakage plating area, and the right side is the complete coating area. The leakage plating area does not form a continuous and dense Fe2Al5 transition layer, which is very loose. Most areas are exposed to the substrate and no transition layer is formed. The transition layer in the complete coating area is relatively dense.
Oxygen can be observed to be significantly enriched in the surface layer of the leakage plating area, and there is obvious oxygen in almost the entire detection area. The last picture is the surface distribution of Zn. The area where Zn is enriched is completely consistent with the tiny zinc particles seen in the first secondary electron phase. Comparing the surface distribution of Zn and Mn, it can be seen that Mn is enriched in the area where Zn is missing, and the corresponding O element is also obviously enriched. Therefore, it can be concluded that the segregation and oxidation of Mn on the surface of the micro-area forms a very thin layer of Mn oxide, which is the key to the failure to form a continuous and dense Fe2Al5 layer and the root cause of the formation of leak plating.

Conclusion: The "chicken claw pattern" defect on the surface of hot-dip galvanized steel plate DP780+Z is a kind of leak plating. Mn elements segregate and oxidize to the surface of the strip during annealing, forming Mn oxides in the micro-area on the surface of the strip. The presence of oxides prevents the formation of continuous and dense Fe2Al5 particles on the surface of the strip after entering the zinc pot, resulting in the inability to form a metallurgical bond between zinc and the substrate, resulting in leak plating.

Large black spot-like defects on the surface of the strip --- damage to the heat exchanger in the cleaning section
Defect description
A type of black scar-like plating defect appears on the surface of the strip.
Features:
1) Size: The defect size is about 3-5cm;
2) Surface: Only on the upper surface;
3) Frequency: Generally, 1-2 defects appear in a steel coil, but once the defect occurs, this type of plating defect will generally appear in 7-8 coils in a row.
4) Morphology: There are two obvious point defects, the larger black one in the middle is plating, and the other is a zinc scar defect.

Macromorphology of plating leakage
Microstructure analysis of plating leakage site
Low-power microstructure morphology characteristics of plating leakage site
Morphology characteristics of the plating leakage site, the defect site is very messy, the zinc layer is a loose grayish white, non-continuous dense structure, there are a large number of loose small holes at the junction of the defect and the normal site, and some of the remaining zinc layer is only mechanically attached to the surface of the strip.
It can be seen that there is no zinc layer attached to the middle leakage plating area. The micro-area composition analysis shows that the composition is mainly composed of O, C, Al, Na, Fe, Zn, etc. The presence of C and O indicates that these parts are enriched with organic dirt, and the Na element indicates that these micro-areas have residual alkaline substances from the cleaning section. The cleaning section mainly uses alkali washing, and the Na element can be used as a signature element for the cleaning section. Fe mainly comes from iron powder remaining on the substrate or surface.

Characteristics of the middle of the leakage plating area and corresponding micro-area composition analysis
Mechanism of leakage plating: For example, the leakage plating in this case can be immediately checked and located based on on-site operation experience. This defect is caused by the damage of the heat exchanger in the cleaning section. According to:
1) Generally, the macroscopic morphology of leakage plating of larger sizes such as 3-5cm is black, and dirt adheres, and the cleaning section can be located immediately;
2) Messy composition characteristics, characteristic element Na, cleaning section.
3) Because the heat exchanger is circulating water inside, generally when the equipment leaks, the dirt brought out after the leakage of the circulating water adheres to the surface of the strip, and finally causes leakage plating. The carry-out of this dirt is a random, discontinuous event. Its characteristic of causing missed plating is that it occurs at a low frequency.