The galvanizing stuff

Definition of electrogalvanizing

Electrogalvanizing, also known as cold galvanizing in the industry, is the process of forming a uniform, dense and well-combined zinc metal or zinc alloy deposit layer on the surface of the workpiece using electrolysis.

Compared with other metals, zinc is cheap and easy to be coated, and it is a low value anticorrosive electrocoating. It is widely used to protect steel parts, especially to prevent atmospheric corrosion, and for various decorative purposes.

The principle of electrogalvanizing

Electrogalvanizing mainly relies on electrolysis to achieve coating. In the plating tank filled with galvanized liquid, the pre-treated workpiece to be plated is used as the cathode, and the anode is made of plated metal, and the two poles are respectively connected with the positive and negative terminals of the DC power supply. Galvanizing solution consists of an aqueous solution containing coated metal compounds, conductive salts, buffers, pH regulators and additives. After electrification, the metal ions in the galvanizing liquid move to the cathode to form a coating under the action of potential difference. The anode metal forms metal ions into the galvanizing solution to maintain the concentration of the coated metal ions.

Classification of electrogalvanizing

Classification by electroplating solution

The product quality of cyanide galvanizing is good, especially color galvanizing, and the color remains good after passivation. However, cyanide is a highly toxic substance and its use has been banned.

The main components of zincate galvanized by NaOH and ZnO and other preparations according to a certain proportion of configuration, belongs to the alkaline additives of zincate galvanized. The crystal structure of the coating is columnar, good corrosion resistance and suitable for color galvanizing.

The main components of chloride galvanizing are KCl and ZnCl and other preparations in a certain proportion. This process is the most widely used in the electroplating industry, suitable for white passivation (blue white, silver white).

The main components of sulfate zinc plating are ZnSO4 and other preparations in a certain proportion. This process is suitable for low cost of continuity (wire, strip, etc.).

Classification by passivation color of electrogalvanizing

Galvanized white passivation, galvanized blue white passivation, galvanized color passivation, galvanized olive green passivation, galvanized black passivation.

According to the composition of passivation liquid is divided into Cr+6 and Cr+3 plating, of which Cr+3 plating is known to us as environmentally friendly galvanized.

Common process flow of electric galvanizing

Cause analysis of coating color difference

After a simple understanding of the principle and process of electrogalvanizing, we will analyze what are the possible reasons for the color difference of electrogalvanizing coating.

◆ Influence of electroplating solution

a. Different electroplating plants, even if the same electrogalvanizing process is selected, but the electroplating solution used will certainly have differences in the solution ratio, which will cause the electrogalvanizing products produced by different electroplating manufacturers will inevitably have color differences.

b. The content of each substance in the electroplating solution is a range value, and even the same electroplating plant cannot guarantee that the composition of the electroplating solution in each ratio is exactly the same. Therefore, the electrogalvanized products of different electroplating batches in the same electroplating plant will also be slightly different in color.

◆ The effect of passivation liquid

Passivation process is the coloring process of electroplating process, but also the decisive process of electroplating color, therefore, the selection and ratio of passivation liquid directly determines the coating color of electrogalvanizing workpiece.

Different electroplating plants, the choice of passivation liquid is different, the color of electrogalvanizing products will inevitably be different.

In the same electroplating plant, the slightly different ratio of passivation liquid in different plating batches will also cause the color difference of electroplating products.

◆ Influence of galvanizing process

In the process of galvanizing, the amount of brightener, temperature, current density, etc., will affect the brightness, uniformity, roughness and other aspects of the coating. These effects will cause certain differences in the visual effect of the electroplating.

Knowledge expansion

◆ Does the coating color difference affect the corrosion resistance of the coating? Many customers will worry about the coating color difference affect the corrosion resistance of the coating, here to tell you that the corrosion resistance of galvanizing is actually determined by the thickness of the coating, as long as the thickness of the coating meets the standard requirements, its corrosion resistance will not be affected, and through the company's laboratory neutral salt spray test verified this conclusion.

◆ The difference between environmental protection galvanized and ordinary galvanized

With the implementation of the European Union's RoHS standard in the world, the pollution caused by heavy metal substances has attracted more and more attention. The passivation solution of ordinary galvanized is mainly Cr+6 heavy chromium salt solution, and Cr+6 is one of the toxic heavy metals prohibited by RoHS. Hexavalent chromium is an ingestible poison, which is extremely toxic when inhaled, and skin contact may cause sensitivity. More likely to cause hereditary genetic defects, inhalation may cause cancer. According to the investigation and research of relevant departments, ordinary electroplating wastewater, hexavalent chromium content of up to 410 ml/l, this wastewater if directly discharged, hexavalent chromium into the soil, with rainfall circulation, will cause serious environmental pollution, even hundreds of years can not be restored. The passivation solution of environmentally friendly galvanized is Cr+3 or chromium-free passivation, and the passivation solution is not harmful to the environment and the human body and meets the requirements of RoHS.

Color difference

Take ordinary color zinc and environmental color zinc as an example:

Ordinary zinc plating color shows obvious yellow and rainbow color, environmental color zinc almost transparent color, in the sun is yellowish rainbow color or lavender.

◆ The relevant implementation standards of fastener plating

The international standard for fastener plating is ISO 4042, and the latest version is ISO 4042:2018

The national standard for fastener plating is GB/T 5267.1, and the current version is GB/T 5267.1-2002 fastener coating

◆ Hydrogen embrittlement risk of electroplating

Electroplating process involves pickling and rust removal process, so there is a large possibility of hydrogen embrittlement, hydrogen embrittlement will cause abnormal failure of fasteners. According to ISO/TR 20491:2019

Fasteners-Fundamentals of hydrogen embrittlement in steel  fasteners with performance grades 10.9 (10) and above and fasteners with Vickers hardness greater than 390HV are not recommended for electroplating.