Electrogalvanizing: It is the process of using electrolysis to form a uniform, dense, and well-adhered metal or alloy deposit layer on the surface of a workpiece.
Compared with other metals, zinc is a relatively inexpensive and easy-to-plate metal. It is a low-cost anti-corrosion electroplated coating and is widely used to protect steel parts, especially against atmospheric corrosion, and for decorative purposes. Plating technologies include vat plating (or rack plating), barrel plating (suitable for small parts), automatic plating, and continuous plating (suitable for wire and strip).
I. Classification of Electrogalvanizing Currently, according to the plating solution type, electrogalvanizing in China can be divided into four major categories: 1. Cyanide zinc plating Since (CN) is highly toxic, environmental protection has imposed strict restrictions on the use of cyanide in electrogalvanizing, continuously promoting the development of low-cyanide and cyanide-free zinc plating bath systems, and requiring the use of low-cyanide (micro-cyanide) plating solutions.
With this process, the plated product has good quality, especially for colored plating, where the color is well maintained after passivation.
2. Zincate zinc plating This process evolved from cyanide zinc plating. Currently, there are two major schools in China: a) The "DPE" series from Wuhan Research Institute of Materials Protection; b) The "DE" series from the Institute of Radio and Television (Guangdian). Both are zincate zinc plating with alkaline additives, with a pH value of 12.5~13.
With this process, the coating crystal structure is columnar, with good corrosion resistance, and it is suitable for colored zinc plating.
Note: After the product is removed from the tank → water rinse → bright dipping (nitric acid + hydrochloric acid) → water rinse → passivation → water rinse → water rinse → hot water rinse → drying → aging treatment (in an oven at 80~90°C).
3. Chloride zinc plating This process is relatively widely used in the electroplating industry, accounting for up to 40%. After passivation (blue-white), zinc can replace chromium (comparable to chromium plating), especially after adding a water-soluble varnish, it is difficult for laymen to distinguish whether it is zinc plating or chromium plating.
This process is suitable for white passivation (blue-white, silver-white).
4. Sulfate zinc plating This process is suitable for continuous plating (wire, strip, simple, large and heavy parts), with low cost.
II. Electrogalvanizing Process 1. Electrogalvanizing process flow Taking zinc-iron alloy plating as an example, the process flow is as follows: chemical degreasing → hot water rinse → water rinse → electrolytic degreasing → hot water rinse → water rinse → strong pickling → water rinse → zinc-iron alloy electroplating → water rinse → water rinse → bright dipping → passivation → water rinse → drying.
2. Preparation of electrogalvanizing plating solution Preparation of the plating solution (using 1L as an example): (1) First add 1/3 volume of pure water to the plating tank;
(2) Use 1/3 of the pure water to dissolve sodium hydroxide (it will generate heat during dissolution, so care must be taken);
(3) Mix zinc oxide with a small amount of water into a paste, then add more pure water and stir thoroughly. Slowly add the stirred zinc oxide to the dissolved sodium hydroxide solution, stirring while adding, and after full complexation, add it to the plating tank;
(4) When the plating solution temperature drops below 30°C, add 85g of Baser and stir thoroughly;
(5) Dissolve 15mL of BaseF in 15g of BaseR, then add the mixture to the plating tank;
(6) Add 4mL of H-0624 and stir thoroughly; add water to the specified volume;
(7) Add brighteners ZF-105A and ZF-105B; stir thoroughly.
Black passivation process flow: water rinse → bright dipping → water rinse → black passivation → water rinse → post-treatment → drying.
3. Factors affecting electrogalvanizing (1) Effect of zinc content If the zinc content is too high, the bright range is narrow, thick coatings are easy to obtain, and the iron content in the coating decreases; if the zinc content is too low, the bright range is wide, it takes longer to reach the required thickness, and the iron content in the coating is high.
(2) Effect of sodium hydroxide If the sodium hydroxide content is too high, burning is likely to occur during high-temperature operation; if the sodium hydroxide content is too low, the throwing power is poor.
(3) Effect of iron content If the iron content is too high, the iron content in the coating is high, and the passivation film is not bright; if the iron content is too low, the iron content in the coating is low, corrosion resistance decreases, and the color tends to be olive.
(4) Effect of brighteners If ZF-100A is too high, the coating is brittle; if too low, there is no coating in the low-current-density area, and the passivation color is uneven; if ZF-100B is too high, the coating is brittle; if too low, the entire coating is not bright.
(5) Effect of temperature If the temperature is too high, the throwing power decreases, the iron content in the coating increases, corrosion resistance decreases, and the passivation film color is uneven and mottled; if the temperature is too low, burning occurs in the high-current-density area, the coating is brittle, and the deposition rate is slow.
(6) Effect of cathode movement Cathode movement must be used. If the movement is too fast, the coating in the high-current-density area is rough; if too slow, gas streaks may occur, resulting in no coating in local areas.
III. Common Faults in Zinc Plating 1. Incomplete degreasing (1) The pretreatment solution temperature is too low; heat it to the range required by the process.
(2) The ultrasonic power is too low or the electrolytic current is too small; adjust to within the process range.
(3) The pretreatment solution is aged; add or replace it as required.
(4) The workpiece has too much oil contamination, and the normal process cannot remove the oil completely. (Reduce the amount of oil used or use qualified rust-preventive oil.) Before racking, use manual wiping to remove most of the oil contamination.
(5) Too much oil floats on the surface of the pretreatment solution, causing the workpiece to re-adsorb oil when lifted out of the solution. The oil on the solution surface should be removed.
(6) The pickling tank and the subsequent water rinse tank are contaminated by oil; they should be cleaned.
(7) The rinse tank after zinc plating or the bright dipping tank and the rinse tank after bright dipping are contaminated by oil; they should be cleaned.
(8) Poor conductivity or no conductivity in the degreasing tank; check whether the racks, copper busbars, flight bars, triangles, anode baskets, and rectifier are conducting properly, and clean or repair them if necessary.
2. Dull coating (1) The current is too low; check whether the current is within the process range and adjust it.
(2) The anode area is too small or uneven, causing partial or local dullness of the workpiece; check the anode condition and distribution and make necessary adjustments.
(3) The brightener content is low; add brightener, but at most 20% of the make-up amount at one time. If it is still not bright, look for other causes.
(4) The zinc oxide content is high, and the cyanide-to-zinc ratio is too low; reduce the zinc oxide concentration or add sodium cyanide to make the cyanide-to-zinc ratio 1.5-2.0.
(5) The substrate surface of the workpiece has dirt, low roughness, or the substrate is corroded; polish or grind the workpiece.
(6) Poor conductivity; check whether the racks, copper busbars, flight bars, triangles, anode baskets, and rectifier are conducting properly, and clean or repair them if necessary.
(7) Intermittent current causes matte etching of the coating; check the operating condition of the rectifier.
3. Burnt workpiece (1) The zinc plating current is too high; adjust it to the normal range.
(2) The workpiece area is too small; adjust the workpiece arrangement to facilitate current adjustment.
(3) The zinc oxide concentration is too low to meet the normal current density requirement; add zinc oxide or increase the zinc anodes, or increase the sodium hydroxide concentration to increase the dissolution rate of the zinc anodes.
(4) The workpiece is corroded during anodic electrolysis; adjust or replace the anodic electrolysis.
(5) Aging of the cathodic electrolytic solution and excessive impurities adsorbed on the workpieces, combined with a poor pickling solution, can produce a burnt-like coating. The cathodic electrolytic solution and the hydrochloric acid pickling solution should be replaced.
(6) Reworked workpieces that are not stripped completely will form a burnt-like coating. The defective coating should be stripped thoroughly before re-plating.
(7) Too many impurities in the hydrochloric acid pickling solution cause a displacement layer to form on the workpiece surface, resulting in a burnt-like coating after zinc plating. The hydrochloric acid pickling solution should be replaced.
4. Trivalent chromium passivation film fogging:
(1) The pH of the passivation solution is too high; adjust it to the process range.
(2) There is no air agitation, or the agitation is too weak; adjust appropriately.
(3) The air hanging time after passivation is too long; adjust appropriately.
(4) The zinc impurity concentration is too high; replace the passivation solution.
(5) The passivation solution temperature is too high; cool it down.
(6) The coating is foggy and not bright; adjust the plating bath.
5. Trivalent chromium passivation film mottling or black spots:
(1) The pH of the passivation solution is too low; adjust it to the process range.
(2) There are too many impurities in the bright dip; replace it.
(3) Oil contamination adheres to the workpieces before passivation; investigate and eliminate it.
6. Color passivation film peeling:
(1) The passivation time is too long; shorten the passivation time.
(2) The pH of the passivation solution is too low; dilute with water.
(3) The passivating agent content is low; add an appropriate amount.
(4) The Cr3+ content in the passivation solution is low; add about 0.5 g/L of zinc powder.
7. Color passivation film fogging:
(1) The passivation solution is aging; replace it partially.
(2) The passivation solution temperature is low; heat it appropriately.
(3) The coating is foggy and not bright; adjust the plating bath.
(4) The passivation solution contains oil; check the compressed air and make adjustments.
8. Trivalent chromium passivation film yellowing:
(1) The iron impurity content in the passivation solution is too high; replace the passivation solution.
(2) There are too many impurities in the bright dip; replace the bright dip solution.
The above is some common and basic knowledge about electrogalvanizing. In daily operation, everyone will encounter various problems. We hope everyone will analyze and summarize more.