The Electrogalvanizing Process at a Glance
Electrogalvanizing, also called zinc electroplating, deposits a layer of metallic zinc onto a steel surface by electrolysis. The steel part or strip acts as the cathode, zinc anodes dissolve into the electrolyte, and an external power source drives the zinc ions onto the cathode surface. Compared with hot-dip galvanizing, electrogalvanized coatings are thinner, typically in the range of 5 to 25 micrometers, but they are much more uniform in thickness, which makes the process attractive for parts that must be assembled with close tolerances. The coating protects the steel in two ways: it acts as a barrier against moisture, and it corrodes preferentially as a sacrificial layer where the coating is damaged.
Electrolyte Composition and Additives
The electrolyte is the heart of the process. Acid zinc baths based on zinc sulfate or zinc chloride are common for both rack plating and continuous strip lines, while alkaline zinc baths are used where throwing power and coating uniformity on complex shapes are critical. The zinc ion concentration, the pH and the temperature of the bath must be kept inside a defined window, because each parameter shifts the deposition behavior. Organic additives such as brighteners and grain refiners control the crystal size and the brightness of the deposit, while carrier agents help the additive system work over a wide current density range. The bath chemistry is maintained by regular analysis and by filtration that removes solid contamination from the solution.
Current Density, Temperature and Agitation
The current density determines how fast the zinc layer grows. Within the usable range of the bath, a higher current density gives a higher deposition rate, so the required coating thickness is obtained in a shorter time. The current efficiency of acid zinc baths is high, often above 90 percent, because most of the current is used for zinc deposition rather than for hydrogen evolution. At the edges and corners of the part the current density is higher than on flat surfaces, so the coating grows thicker at these locations, and this effect must be considered when the minimum thickness requirement is specified. Temperature influences the conductivity of the electrolyte, the solubility of the zinc salts and the activity of the additives; most acid zinc baths operate in a moderate range, for example 40 to 60 degrees Celsius for sulfate baths, and the bath is cooled or heated because the process itself generates heat. Agitation by air, pump circulation or cathode movement supplies fresh zinc ions to the surface and sweeps away hydrogen bubbles that would otherwise form pinholes, and uniform agitation is essential on large flat parts where a stagnant layer of solution would limit the deposition rate.
Substrate Condition and Pre-Treatment
The steel surface must be clean and active before plating. Rolling oil, rust and oxide films prevent the zinc layer from adhering, so the parts are degreased, rinsed and pickled in sequence. The surface roughness of the substrate influences the adhesion and the appearance of the deposit, and a properly activated surface produces a fine-grained, adherent zinc layer. Immediately after plating the parts are rinsed and, where required, passivated or sealed to stabilize the surface and to control the formation of white rust during storage and transport.
Quality Control and Testing
The coating mass is the main acceptance criterion and is determined gravimetrically by dissolving the zinc layer from a weighed sample, in accordance with ISO 2081 and the corresponding national standard GB/T 9799 for electroplated zinc coatings. The adhesion of the coating is checked by bending or forming tests, and the corrosion resistance is evaluated by neutral salt spray testing according to ISO 9227. The coating thickness can be measured non-destructively with magnetic gauges on the finished part. Each production batch should record the bath analysis, the plating time, the current and the coating mass results so that the process remains under statistical control.
Frequently Asked Questions
Q: What is the difference between electrogalvanizing and hot-dip galvanizing?
Electrogalvanizing deposits a thin and very uniform zinc layer, typically 5 to 25 micrometers, through electrolysis. Hot-dip galvanizing produces a much thicker coating by immersion in molten zinc and gives longer corrosion protection, but the thickness is less uniform and the surface has a different appearance.
Q: Why are additives needed in the zinc bath?
Additives such as brighteners and grain refiners control the crystal structure and brightness of the deposit, extend the usable current density range, and improve the uniformity of the coating over the part surface.
Q: How is the zinc coating thickness measured?
The thickness can be measured non-destructively with magnetic induction gauges on finished parts. For verification of the coating mass, a gravimetric method is used in which the zinc layer is dissolved from a weighed sample and the mass difference is calculated.
Q: What causes white rust on electrogalvanized surfaces?
White rust forms when the zinc surface is exposed to moisture for a prolonged period and the corrosion products accumulate. It can be minimized by passivation or sealing after plating and by proper drying and packaging during storage and transport.
Q: Can electrogalvanized parts be painted?
Yes. The zinc surface provides a good base for paint systems after suitable pre-treatment, and the combination of zinc and paint gives significantly better corrosion protection than either system alone. The paint supplier should be consulted for the correct pre-treatment sequence.
Q: Which standards apply to electroplated zinc coatings?
ISO 2081 and its national equivalent GB/T 9799 specify the requirements for electroplated zinc coatings on iron and steel, including coating designations and thickness classes. Neutral salt spray testing is carried out according to ISO 9227.