Bath for direct chrome-plating of calender rolls

An electrolytic bath for direct chrome-plating of calender rolls is disclosed to which potassium aluminum sulfate and copper sulfate are added in order to increase the strength and stability of the chrome-plated layer deposited on the calender rolls.

Calender rolls, especially those intended for the production of plastic 
foils, must be chrome-plated to give them a shell surface as smooth and 
uniform as possible. Traditionally, the chrome layer is applied by direct 
galvanizing, i.e., without any intermediae layer on the basic material of 
the rolls (steel or cast iron). Galvanic baths are used for this purpose, 
containing chromic acid (CrO.sub.3) and sulfuric acid (H.sub.2 SO.sub.4) 
in a water solution (so-called "chromic acid electrolytes") having 
additional contents of fluosilicic acid (H.sub.2 SiF.sub.6) to improve the 
deposit speed (so-called "mixed acid electrolytes"). 
It has been found that sufficient stability and strength in the 
chrome-plating cannot be attained with the known baths. The chrome layers 
which may be 30 to 300 .mu.m thick appear to be perfect when the roll is 
taken from the bath, but at the first use of the roll in the calender, we 
often find surprising and despite previous testing unforeseen formation of 
macro-cracks. These cracks make the roll useless, since they result in 
defective foils. The respective rolls must, therefore, be removed from the 
calender and replaced by new rolls, which involves considerable expense. 
The waste quota is relatively high, thus putting a marked additional 
burden on the production cost of the rolls. 
Direct chrome-plating of iron or steel objects has actually been known for 
a long time. However, the processes so far applied successfully to 
chrome-plating of other objects cannot be equally applied to 
chrome-plating of calender rolls, since the chrome layer on a calender 
roll must withstand considerable stress of temperature and pressure as 
well as shearing stress not present in other objects. For other chromed 
objects it is usually only important to produce a corrosion protected and 
an aesthetically acceptable surface with the chrome-plating, so that the 
processes there applied do not give us any indication as to how to improve 
the chrome-plating of calender rolls. 
Consequently, to this date we have accepted the high waste quota of 
chrome-plated calender rolls as inevitable. But the present invention 
eliminates this inconvenience. In said invention, a galvanic bath is 
provided which contains the usual components of chromic acid electrolytes 
and mixed acid electrolytes for direct chrome-plating of calender rolls 
and which is distinguished as a novel invention by its additional contents 
of copper sulfate (e.g., CuSo.sub.4.5H.sub.2 O) and potassium aluminum 
sulfate [e.g., KAl(SO.sub.4).sub.2.12H.sub.2 O]. In this case, we prefer 
to use about 0.3 to about 0.5 weight percent (wt.%) of potassium aluminum 
sulfate and up to about 0.1 weight percent (wt.%) copper sulfate, each 
referred to the contents of chromic acid. 
Surprisingly, it was discovered that with the bath according to this 
invention, at essentially the same other electrolytic conditions, a much 
more stable chrome layer can be produced on the calender rolls and that in 
particular the tendency towards macro-cracks, which has been frequent 
during the first use of the rolls and could not be recognized in advance, 
vanished to a large extent. This lowers the waste-quota considerably, and 
the production of the calender rolls is less expensive. This function of 
the additions of copper sulfate and potassium aluminum sulfate to the 
electrolytic bath could neither be anticipated nor can it be explained. 
We found that it is advisable, as compared to the customary chromic acid 
electrolytes, or mixed acid electrolytes containing about 2.5 weight 
percent (wt.%) sulfuric acid, to reduce the content of the sulfuric acid 
an amount equal to the SO.sub.4 -- ions which get into the bath from the 
copper sulfate and potassium aluminum sulfate, i.e., by an amount to keep 
the SO.sub.4 -- ion concentration of the bath essentially the same as that 
of a regular bath. This enhances the effect of the addition of copper 
sulfate and potassium aluminum sulfate.

EXAMPLE 
The invention is explained hereafter in detail in one example of execution. 
A galvanic bath of the following composition has been made: 
250 g: chromic acid 
1.0 - 1.25 g: potassium aluminum sulfate 
0.25 g: copper sulfate 
2.0 g: sulfuric acid, free of water 
6 ml: fluosilicic acid commercial type (about 35%) 
add 1 1: water. 
With this bath, calender rolls were chrome-plated at a temperature between 
about 50.degree. to 60.degree. C and at current densities of about 20 and 
100 A/dm.sup.2. The duration of the chrome-plating was determined by the 
desired thickness of the layer and density of the current, whereas the 
density of the current was on the other hand influenced by the size of the 
work piece. The chrome layer was deposited as usual, without an 
intermediate layer, directly onto the steel or cast iron, keeping the 
thickness of the layer within the usual range of about 30 to 300 .mu.m. 
In many chrome-platings made at these conditions, markedly stable chrome 
layers were obtained which led to less than 2 percent (%) waste in all 
calender rolls chromeplated. 
For comparison, calender rolls were chrome-plated in a usual bath which was 
distinguished from the abovementioned in that it did not contain any 
copper sulfate nor potassium aluminum sulfate and that 2.5 g sulfuric acid 
were used instead of 2 g. The other process conditions were the same. The 
chrome layers thus obtained were much less stable, the waste rate of the 
thus-produced calender rolls was around 20 percent (%). The invention 
therefore lowers the waste rate by a power of 10.