Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 190 of 707

a third bath is a mixed catalyst bath that also uses sulfate but the secondary cat- alyst is a highly stable organic compound based on sulfur chemistry. Fluoride is typically not used in this type of bath except in special circumstances. The primary catalyst for all types of baths remains to be sulfate. Without sulfate no chrome will deposit. The ordinary or basic formulation is still used in many applications. It is cheap and easy to maintain but does not give the enhanced physical character- istics that the mixed formulations give. In the ordinary bath the efficiency is between 7 to 15%. This means that from 93% to 85% of the electrical energy is going to generate hydrogen and oxygen as gases as shown in the basic reaction section and only 7 to 15% is going to deposit chrome. Another type of chemistry is the mixed catalyst baths that contain a secondary catalyst system. The first of these baths was introduced in the 1950's. These baths use a fluoride type catalyst in addition to the sulfate to give faster deposit rates due to its higher efficiency. The fluoride or mixed catalyst baths were first introduced as Self Regulating High Efficiency (SRHS). They were able to con- trol sulfate catalyst and the fluoride catalyst based on temperature adjust- ments. In the early days of its development these baths could operate as a dec- orative bath by day and a hard chrome bath by night to plate thicker deposits. The major drawback to these and all fluoride baths is that they are more aggres- sive and etch, or attack, unprotected parts of the work. To use these types of baths, extensive masking was required so unprotected work would not be etched or destroyed. While the fluoride chemistry was used in decorative appli- cations as well, the short duration of plating did not give as bad an effect as the longer plating time used in functional chrome. This was a major drawback when the ID of work was not protected or masking required a lot of labor. The attack of steel or copper substrate from the fluoride etching also contributed sig- nificantly to impurities in the bath and made bath control difficult. It was not until the late 1980's that a new type of catalyst was introduced that did not cause etching. The new chemistry used chromic acid and sulfate but now used a highly stable sulfur-based catalyst. The new bath was no more aggressive than an ordinary sulfate solution. This made the control of the bath significantly eas- ier; reduced deterioration of fixtures, tank linings and allowed for less expensive equipment for heating and cooling. It also gave a superior deposit in terms of phys- ical qualities. The deposit was harder and showed improved corrosion resistance. CATHODIC REACTIONS The deposition of chrome metal from the hexavalent state is a complex reaction but for a practical working knowledge there are three basic reactions at the cathode that allow the deposit of chrome metal to take place. First, however, we need to clarify that powered "chromic acid" is really chromium trioxide (CrO3) and does not become chromic acid (H2CrO4) until dissolved in water. 2CrO3 + 2H2O j 2H2CrO4jH2Cr2O7 + H2O (Chromium Trioxide to Chromic acid) (Cr2O7)-2 + 12 e- + 14H+j2Cr0 + 7H2O (metal is formed) 2H+ + 2e-jH2 (Generation of hydrogen gas) (Cr2O7)-2 + 6 e- + 14H+j2Cr+3 + 7H2O (Formation of Cr (III)1) 189

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