Method for detecting the presence of a chromate coating on aluminum

A method is disclosed for detecting the presence of a chromate coating on the surface of aluminum. The method includes the application of a few milliliters of a liquid composition to the surface of the aluminum. The liquid composition includes one part of potassium permanganate having a concentration of at least 0.5 weight/volume percent and one part of concentrated sodium hydroxide saturated in an aqueous solution. The liquid has a beet-red pigmentation which undergoes a color change to a bright blue-green pigmentation when it reacts with a chromate coating. If no coating is present on the aluminum surface, the liquid will react with the aluminum and the alloys contained therein to produce a beige color.

FIELD OF THE INVENTION 
The present invention relates to a method for the detection of the presence 
of a chromate coating on the surface of aluminum. 
BACKGROUND OF THE INVENTION 
Currently, many engine blocks and engine parts are made of aluminum to 
provide a lighter weight vehicle. When using aluminum, it is necessary to 
treat the surface with chromic acid to remove oxides so as to improve 
paint adherence. In so doing, a very faint light gold appearance is 
obtained on the aluminum part. However, in today's modern factories, 
wherein sodium vapor lights are used, it is sometimes very difficult for a 
person to visually detect the presence of a chromate coating. Up until 
now, the presence of such a coating could only be positively identified by 
a very time consuming analysis of atomic absorption. Therefore, there is a 
desire in the industry to provide a fast, on-the-spot and reliable means 
of detecting the presence of a chromate coating on aluminum parts which 
are to be painted. 
Now a method and composition have been invented for detecting such a 
coating. 
SUMMARY OF THE INVENTION 
Briefly, this invention relates to a method for detecting the presence of a 
chromate coating on an aluminum surface. The method includes the 
application of a few milliliters of a liquid solution comprising one part 
of potassium permanganate having a concentration of at least 0.5 percent 
weight per volume (0.5 grams of KMnO.sub.4 per 100 milliliters of H.sub.2 
O) and one part of concentrated sodium hydroxide. The liquid solution has 
a beet-red pigmentation and reacts on the aluminum surface such that a 
color change will be produced. During the color change, the solution will 
change from a beet-red pigmentation to a bright blue-green pigmentation 
when a chromate coating is present and will change from a beet-red 
pigmentation to a beige color if no chromate coating is present. 
The general object of this invention is to provide a method for detecting 
the presence of a chromate coating on the surface of aluminum. A more 
specific object of this invention is to provide a fast, on-the-spot and 
reliable test for detecting a chromate coating on an aluminum surface. 
Another object of this invention is to provide a simple and inexpensive 
method of detecting the presence of a chromate coating on the surface of 
aluminum. 
Still further, an object of this invention is to provide a composition of 
matter which is useful in detecting the presence of a chromate coating on 
the surface of aluminum by visually changing colors. 
Other objects and advantages of the present invention will become more 
apparent to those skilled in the art in view of the following description. 
DETAILED DESCRIPTION OF THE INVENTION 
In accordance with this invention, a liquid composition is prepared for use 
in detecting the presence of a chromate coating on the surface of an 
aluminum part, such as the aluminum block of an engine. The liquid 
composition is made up of one part of potassium permanganate having a 
concentration of at least 0.5 weight per volume percent and one part of 
concentrated sodium hydroxide. Preferably, the concentration of the 
potassium permanganate is between 0.5 and 1.0 weight per volume percent 
(0.5 and 1.0 grams of KMnO.sub.4 per 100 milliliters of water) and the 
concentrated sodium hydroxide is a saturated aqueous solution. 
The method for detecting the presence of a chromate coating on the surface 
of aluminum includes the steps of applying a couple of drops of the 
aforementioned liquid composition to a level surface of the aluminum part. 
The liquid will have a beet-red pigmentation since potassium permanganate 
is beet-red in color while concentrated sodium hydroxide is clear in 
color. The clear sodium hydroxide does not change the beet-red 
pigmentation of the potassium permanganate when the two are combined. 
Although equal amounts of the potassium permanganate and the sodium 
hydroxide can be combined to form the liquid composition, it should be 
noted that best results are obtained when the sodium hydroxide exceeds the 
potassium permanganate and preferably the ratio is three parts of sodium 
hydroxide to two parts of potassium permanganate. The presence of a 
greater amount of sodium hydroxide with the potassium permanganate speeds 
up the reaction. When the liquid composition is applied to a surface of 
aluminum having a chromate coating, the liquid composition will react with 
the coating to produce a color change in which the beet-red pigmentation 
will change to a bright blue-green pigmentation. If no chromate conversion 
is present, the liquid composition will react with the aluminum and the 
alloys contained therein to form a dull brown or beige color. It should be 
noted that the concentrated sodium hydroxide is very caustic and acts as a 
catalyst to allow the reaction to progress between the aluminum and the 
potassium permanganate. 
As explained above, a liquid composition comprising equal parts of 
potassium permanganate and concentrated aqueous sodium hydroxide can be 
applied to an aluminum part to detect the presence of a chromate coating. 
However, by experimentation, it has been found that by first applying 0.1 
milliliter of potassium permanganate having a concentration of at least 
0.5 percent to an aluminum surface and then applying 0.15 milliliter of 
concentrated sodium hydroxide to the potassium permanganate that the 
method also works. Within a short period of time, approximately 30 to 60 
seconds, the liquid composition will react with the aluminum surface and a 
color change will occur. The colors will be the same as explained above. 
Likewise, the method for detecting the presence of a chromate coating on 
an aluminum surface will work if the sodium hydroxide is first applied to 
the aluminum surface and then the potassium permanganate is combined with 
it.

The following example is set forth to further illustrate the advantages and 
utility of the present invention. 
EXAMPLE 
The following method was used to detect the presence or nonpresence of a 
chromate coating on several specimens of aluminum parts, some of which 
contained a chromate coating and some of which did not. Visually, one 
could not distinguish the parts apart. First, approximately 0.1 milliliter 
of potassium permanganate, having a concentration of between 0.5-1.0 
weight/volume percent was applied to a small surface area on the aluminum 
parts. Immediately thereafter, 0.15 milliliter of concentrated sodium 
hydroxide saturated in an aqueous solution was applied to the potassium 
permanganate. The combined liquid composition had a beet-red color. 
Approximately 30 to 60 seconds after the solution was applied to the 
aluminum surfaces which contained a chromate coating, the color changed 
from beet-red to a bright blue-green. Tests conducted on aluminum parts 
that did not have a chromate coating resulted in a color change from 
beet-red to beige or dull brown. The beige or dull brown indicated that 
the liquid composition had reacted with the aluminum and the alloys 
contained therein. 
While the particular details set forth above are capable of attaining the 
objects and providing the advantages herein stated, the specific materials 
described and the method disclosed are merely illustrated and could be 
varied through the use of other oxidizers and catalysts. Accordingly, this 
invention is intended to embrace all techniques which produce the same 
results without departing from the scope of the inventive concept as 
defined in the appended claims.