Water is often referred to as the “universal solvent.” Yet, as we all know, its hard to get grime off of your hands clean without using some soap! Water, however, is an attractive “solvent” as it is plentiful, relatively cheap and safe. The problem is that water just isn’t good at getting things clean all by itself. The good thing about water is that although it is not a good solvent for many contaminants that need to be removed from parts, it is pretty good at dissolving other things which improve its solvent characteristics.
Three types of water-based cleaning chemistries are alkaline, acid and a third variety (which really isn’t a solution) called an “emulsion.” Nearly all cleaning chemistries also include ingredients in addition to the main constituent which may enhance or in some other way tailor its overall performance. Let’s start off by looking at acid type cleaners.
Acid Based Cleaning Chemistry
Acidic cleaning chemistries are probably the least frequently used of the three characteristic groups mentioned above when it comes to parts cleaning. Acid cleaners often clean by initiating a chemical reaction with the contaminant that is being removed. A good example of an acidic cleaner is a formulation designed to remove rust from a steel part. The acid reacts with the iron oxide (rust) to remove it from the steel part. “Cleaning” is a borderline term for such uses. At the initiation of this blog nearly two years ago, we defined “cleaning” as removing a contaminant from a surface without changing the characteristics of the surface. In most applications using an acidic cleaner, there is a little more going on than just removing a contaminant. Acidic cleaners, for example, are used extensively in the plating industry to “pretreat” surfaces prior to plating. Although the acid cleaners often contain ingredients which also remove such things as oil and some soluble contaminants, the real purpose in using an acidic chemistry is to remove surface oxides from the substrate to provide an “active” and sometimes “etched” surface to facilitate the plating process. This effect falls, technically, outside the realm of “cleaning.” If the only goal was to remove the contaminants (and not condition the surface for plating), non-acidic chemistry would probably be equally effective.
Acids are also used in applications where the effect of an acid results in other desired effects. One of these applications is in the “passivation” of medical prosthesis such as knees, hips, pacemaker housings and the like. Although commonly called “passivation,” one of the major goals in this application is to kill and remove residues of any biologically active contaminants or “pathogens.” Again, it may be a bit of a stretch to call this a “cleaning” application.
The more prevalent main constituents of acidic chemistries are sulfuric, hydrochloric, acetic, citric and phosphoric acids. These acids are usually used in relatively low concentrations in formulations that also contain wetting agents and other ingredients to provide a desired effect. Since acids are generally aggressive against many substrates, it is not uncommon to use “inhibitors.” “Inhibitors” are ingredients which limit the activity of the acid to prevent excessive etching of the parts being “cleaned” or to extend the life of the vessels used to contain the acidic formulation.
Finally, the effectiveness of acidic chemistries, like any others, is dependent on a number of variables which include time, temperature and concentration. Any chemistry should be used according to the manufacturer’s recommendations.
– FJF –