Titanium Coils for Chromium Plating Baths

Titanium Coils for Chromium Plating Baths

Chromium-plating baths mainly consist of aqueous solutions of chromic acid with minor additions, e. g. of sulphuric acid, chromic sulphate, chromic chromate, or fluorsilicic acid. They are regarded as strong oxidation agents. The operating temperature is between room temperature and about 65°C.

Chromium-plating equipment often consists of steel trays lined with wire-reinforced glass plates or with antimonious lead. In some cases, it is possible to use brick-lined trays. Due to the high amperage, the chromium-plating bath is constantly heated and the coils which firstly were required for heating up can now serve as cooling coils. The tubing for external heating up the water bath is made of steel. Although more favorable, internal heating is difficult because the service life of many conventional materials in a chromium-plating bath is too short. Up till now, these heating and/or cooling coils have been manufactured of lead alloyed with tin or antimony. But these did not show a fully satisfying corrosion behavior.

Since titanium is about 2. 5 times lighter than lead but has a much higher tensile strength, heating coils can be produced of titanium much easier, simpler, and without any supporting structures. The corrosion resistance of titanium to chromic acid is outstanding in the usual concentration and temperature ranges. Heating coils made of titanium and exposed to chromium-plating solutions containing 25 % chromic acid do not show the least corrosion stains even after several years of operation.

The materials coming into question for heating coils are the two softer commercially pure titanium grades permitting easy cold working and welding. Heating coils can, without difficulties, be made of welded tubes because the corrosion properties of titanium welding joints are entirely identical to those of unwelded sheets.

Standard heating and cooling coils of titanium manufactured in the dimensions 25. 4 mm outer dia. x 0. 9 mm wall thickness x 6. 100 mm length having a heating area of about 0. 5 m 2. If a larger heating area is required several standard coils can be connected to each other by rubber or plastic hoses. This arrangement permits an exchange of only the damaged coil, in case of trouble or for repair.