Aluminum Cleaning

Q. I am a process engineer who is new to precision cleaning. Can you provide me with an article or information that goes into depth about how to clean different grades of aluminum? I understand there are many different factors that come into play, like type of contamination or cleanliness grade, but I’m interested in how the different grades of aluminum affect the cleaning process. –J.R.

A. You have touched on an important point: The cleaning process needs to be tailored to both the base materials and the type of soils being removed in order to be effective. A deep-drawing compound will have a different chemistry than a machining coolant. Similarly, a mild steel will present different cleaning challenges than an aluminum alloy.

Wrought and cast aluminum alloys are broken into families based on their primary alloying element(s). The wrought designations are four digit numbers with the first digit (1-8) signifying the specific group to which the alloy belongs. The Aluminum Association is responsible for the assignment of the alloy within the general family. A quick summary of those alloy designations are as follows:

•  1000 series – This is essentially unalloyed aluminum containing 99 percent aluminum with minor amounts of iron and/or silicon.

•  2000 series – The primary alloying element is copper, which allows for significant precipitation hardening with the correct heat treatment.

•  3000 series – This group can be work-hardened and has manganese as its primary alloying element.

•  4000 series – This group is unique in that it is alloyed with silicon that significantly reduces the melting range, making its primary use for brazing other aluminum alloys.

•  5000 series – This group uses magnesium as the primary alloying element and is work-hardenable.

•  6000 series – There are two elements that characterize this group: silicon and magnesium. This combination, along with the proper heat treatment, makes this alloy precipitation-hardening.

•  7000 series – This group is also precipitation-hardening and its primary alloying element is zinc

•  8000 series – This group is not as clearly defined and contains numerous alloys with iron and others with lithium.

The above summary is useful to distinguish the varied types of aluminum materials and forms, however, there are not that many differences in the types of cleaning methods for each of them. In general, most metal cleaning starts with a degreasing process, otherwise any subsequent processing (phosphating, etching, etc.) will be ineffective or uneven. In cleaning the aluminum, you could generally go down two paths: solvent or aqueous methods. Again, the type of lubricant and post-process applications will dictate the specific cleaning process.

If heavy lubricants are present, from a deep-drawing operation, for instance, solvent cleaners may offer a better chance at effective removal. If lighter lubricants or coolants are present, aqueous cleaning may be a more effective means. In general, with aqueous cleaning of aluminum, you will need to utilize either a light-duty, mildly alkaline cleaner (pH ~7-9), or a higher-pH cleaner (~10-12) formulated with silicates (usually sodium metasilicate). 

Following degreasing, there may be a need to etch the aluminum if it is to be plated, to provide a matte finish or simply to reestablish a uniform oxide thickness. In this-case, high-pH, uninhibited alkaline etchants will be effective at accomplishing this. It is common for aluminum to develop a smutted surface as a result of dissolution from the high-pH etchant solution without some of the alloying elements in it also dissolving. In this case, the category of aluminum alloy can have some effect on the amount and tenacity of the residual smut. For instance, the low number of or no alloying elements found in the 1000 series means that, despite an aggressive etching, it will have little or no residual smut. In general, the more highly alloyed the aluminum, the greater the amount of smut from the etching process.

In order to remove the residual smut, most aluminum alloys will go through a room-temperature nitric acid bath. This is where the type of alloying element will tend to reveal how difficult it can be removed. The nitric acid works well with copper and zinc alloying elements, but one of the more difficult to remove in nitric acid is a silicon-alloyed aluminum.