The nature and use of cadmium plating
Cadmium plating specific methods
Cadmium plating is mainly used as a protective layer of steel parts. In general atmospheric and industrial atmospheres, the cadmium coating is a cathodic coating relative to the steel substrate, whereas the cadmium coating is anodic coating in humid atmospheres or marine atmospheres that do not contain industrial impurities. The wider application of cyanide bath plating solution, ammonia carboxylamine complex plating solution and acid bath.
Cyanide cadmium plating
Cyanide bath has a greater role in cathodic polarization, coating smooth and detailed. When the additive is contained, a bright coating is obtained, which has few voids and high corrosion resistance, but tends to generate hydrogen embrittlement. Mainly suitable for lower tensile strength steel protection.
a. Bath 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 be added according to the quality of the coating.
b. When using the anode should be sheathed jacket, when not in the anode should be removed. Copper and solder the best parts under the slot, to avoid iron, copper, tin and other impurities increased.
c. The allowable impurity content of the plating solution is not more than 1g / L for copper, not more than 0.05g / L for tin, not more than 10g / L for iron and aluminum and not more than 80g / L for carbonate. Metal impurities with a small current treatment, the removal of carbonate with cyanide zinc.
Acid complexing agent cadmium plating
Ammonia carboxylange complex plating solution can obtain fine crystal and good corrosion resistance of cadmium plating solution stability, both plating ability and deep plating capacity is no less than cyanide plating cadmium plating solution. However, the coating brittle, the solution of serious corrosion of equipment, sewage treatment more difficult.
Acidic cadmium plating
Acid plating bath bath composition is simple, low cost. Because there is no added complexing agent, the role of smaller cathode polarization, both plating capacity and poor plating capacity, only for plating simple shape parts.
The nature and use of cadmium plating
The chemical properties of cadmium and zinc coating similar to the surface in humid air to generate a layer of basic carbonate film, has a protective effect, can protect the layer of cadmium so that it will not continue to be corroded.
Like cadmium plating and zinc plating, tend to produce "hydrogen embrittlement" on steel, especially high strength steel parts, but the tendency of cadmium plating to produce hydrogen embrittlement is smaller. Therefore, low hydrogen-hydrogen embrittlement of cadmium is usually used as an important protection method for structural steel, high-strength steel and spring steel components. However, after electroplating, rigorous dehydrogenation treatment is still required, or other more effective hydrogen permeation prevention measures can be adopted.
Cadmium is also prone to "cadmium brittle" phenomenon, usually at higher temperatures, and at the same time there is a certain stress conditions, can make cadmium and steel cadmium brittle. As the cadmium plating layer above 232 ℃ can be etched into the steel substrate, can cause the so-called "cadmium crisp" phenomenon, so the use temperature should not exceed 232 ℃.
Cadmium plating can be welded, the plating can reduce the oxidation of copper contacts, but also does not increase the contact resistance. Cadmium plated layer is usually used in steel fasteners, pipe fittings and other components, aluminum and aluminum alloy parts, magnesium alloy parts and rubber and steel parts in contact with the pieces. In particular, some elastic parts, threaded parts, standard parts and aerospace, shipbuilding, electronics and military products, such as the habit of using cadmium plating. However, the cadmium vapor and its soluble salts are highly toxic and seriously pollute the environment. Therefore, the application of cadmium plating is subject to strict restrictions, is now rarely used, most of the zinc coating and zinc alloy layer to replace the cadmium plating layer.
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