What Is Involved in Cadmium Plating?

Cadmium plating is widely used to protect steel and cast iron from corrosion. The inner layer of metal is protected by cadmium plating. Since cadmium is an anode relative to iron, the bare metal below it can be protected by the consumption of cadmium plate even when cadmium is strained or cracked. Cadmium is generally used as a thin layer (less than 25 m or 1 mil thick) for atmospheric corrosion resistance.

By electrolysis. Method for depositing a layer of cadmium on the surface of steel parts. It is used to prevent the corrosion of seawater or salt spray, improve the tightness and fit of the assembly, and increase the appearance. It is widely used in the manufacturing industry of marine parts, marine instruments, instruments and fastening parts. It is divided into acid cadmium plating method (with cadmium sulfate or cadmium fluoroborate as the main component of the electrolyte) and cyanide cadmium plating method (with cyanide Nickel, sodium hydroxide and sodium cyanide are the main components of the electrolyte). The cyanide cadmium plating electrolyte is simple to prepare, has a high distribution capacity, and can be used to plate parts with complicated shapes. The cadmium coating has fine and strong bonding crystals, and has many applications.

Cadmium is relatively stable and has good corrosion resistance in parts exposed to marine atmosphere or seawater and hot water above 70 ° C. Therefore, cadmium plating is often used in marine environments.

Cadmium is a silver-white plastic metal. Cadmium metal is easy to forge and roll. The relative atomic mass of cadmium is 112.41, the density is 8.65 g / cm ³ , the melting point is 320.9 ° C, and the boiling point is 767 ° C. Cadmium is present as a divalent metal in compounds. Its hardness is 200HV. The chemical properties of cadmium are similar to zinc, but it is insoluble in lye, soluble in nitric acid and ammonium nitrate, and slowly dissolved in dilute sulfuric acid and dilute hydrochloric acid. At room temperature and dry air, almost no chemical changes occur, but cadmium is easily oxidized in humid air, and the formed oxide film can protect metal cadmium from further corrosion.

Cadmium vapor and soluble cadmium salts are toxic, and cadmium pollution must be strictly prevented. The standard potential for cadmium is -0.4V. In the general industrial atmospheric environment, cadmium plating is a cathodic coating on steel parts, but in marine climate, seawater or high temperature environments, cadmium plating is an anode coating on steel parts. When the working temperature exceeds 232 ° C, the cadmium coating is liable to cause brittleness.

Cadmium coatings are commonly used for the protection of aerospace, marine, radio and electronic product parts.

The types of cadmium plating solutions used in domestic production are: ammonia carboxylate complex cadmium plating, acid sulfate cadmium plating, and cyanide cadmium plating. Beside this there is

Cadmium coating is mainly used as a protective layer for steel parts. Under general atmospheric and industrial atmospheric conditions, cadmium plating is a cathodic coating, as opposed to steel substrates. Under humid or marine atmospheric conditions that do not contain industrial impurities, cadmium plating is an anode coating. Widely used plating solutions include cyanide plating solution, aminocarboxyl complexing agent plating solution and acid plating solution.

Cyanide cadmium plating

The cyanide bath has a large cathodic polarization effect, and the coating is smooth and fine. When an additive is contained, a bright plating layer can be obtained, which has few pores and high corrosion resistance, but has a high tendency to generate hydrogen embrittlement. Mainly suitable for the protection of rhyme steel with low tensile strength.

a. The plating solution should be regularly analyzed, adjusted and filtered. According to the analysis results, sodium cyanide and cadmium oxide were added to control NaCN: Cd = 3 4: 1. Other ingredients should also be added according to the quality of the coating.

b. Cover the anode when using the anode, and take it out when not in use. Copper and soldered parts are preferably charged under the groove to avoid the increase of iron, copper, tin and other impurities.

c. The impurity content allowed in the plating solution is: copper is not more than 1 g / L, tin is not more than 0.05 g / L, iron and aluminum are not more than 10 g / L, and carbonate is not more than 80 g / L. Metal impurities are treated with a small current, and the removal of carbonate is the same as that of cyanide zinc plating.

Amino acid complexing agent cadmium plating

Ammonia carboxyl complexing agent plating solution can obtain cadmium plating layer with fine crystals and good corrosion resistance. The solution is stable. but

Electroplated cadmium molded parts

The chemical properties of cadmium are similar to those of galvanized layer. It can form a basic carbonate film on the surface in humid air, which has a certain protective effect and can protect the cadmium layer from further corrosion.

Like zinc plating, cadmium plating tends to cause "hydrogen embrittlement" on steel, especially high-strength steel parts, but cadmium plating has a lower tendency to generate hydrogen embrittlement than zinc plating. Therefore, low hydrogen embrittlement cadmium plating is usually used as an important protection method for structural steel, high strength steel and spring steel components. However, after electroplating, strict dehydrogenation treatment or other more effective hydrogen permeation prevention measures are still required.

Cadmium plating is also prone to "cadmium embrittlement", usually at higher temperatures and under certain stress conditions, which can cause cadmium embrittlement in steel and titanium alloys. Because the cadmium plating layer can be etched into the base of steel parts above 232 ° C, which can cause the so-called "cadmium embrittlement" phenomenon, the use temperature should not exceed 232 ° C.

The cadmium layer can be soldered. This plating can reduce the oxidation of copper-plated electrical contacts without increasing the contact resistance. Cadmium plating is usually used on steel fasteners, piping parts and other stress parts, aluminum and aluminum alloy parts, magnesium alloy parts, and steel and copper parts in contact with rubber. In particular, some elastic parts, threaded parts, standard parts, and aerospace, shipbuilding, electronics, and military products are used to adopt cadmium plating. However, cadmium vapor and its soluble salts are highly toxic and seriously pollute the environment. Therefore, the application of the cadmium plating layer is severely restricted, and has been rarely used. Most of them use a zinc plating layer and a zinc alloy layer to replace the cadmium plating layer. ^[4]