Q. We are removing light to heavy rust. Is there a reverse-current cleaner that will blow and clean off the rust? Can this be done in a timely manner? I currently deal with some caustics and acids and would like to find a product that is environmentally friendly. J.J.
A. There are two general modes of electrolytic cleaning: anodic and cathodic. In both cases a DC current is applied between an insoluble electrode and the workpiece to be cleaned. When doing anodic electrocleaning, the workpiece is made the anode in the system. This is generally performed in a solution with a basic pH to facilitate the oxidation reaction 4(OH)- → 2H2O + O2(g) + 4e-. In this case, oxygen gas bubbles are generated directly at the workpiece, beneath the contaminant, which greatly helps lift and remove the rust, deposits and light oils.
The other mode is referred to as cathodic electrocleaning, where the workpiece is made the cathode and a reduction reaction occurs at the surface. In this case, the pH of the system needs to be acidic to provide sufficient hydrogen ions to sustain the reaction 4H+ + 4e- → 2H2(g). This mode of electrocleaning effectively cleans in the same manner by lifting deposits like rust or oxide from the metal surface.
In either case, the cleaners need to be basic and acidic, respectively. However, they do not need to be highly caustic or acidic in order to be effective. The bubbling and mechanical action that can occur as a result of an extreme pH cleaning can also be accomplished with the assistance of a direct-current power source at a less severe pH.
In the case of cathodic electrocleaning, the system is said to be more efficient because twice as much hydrogen is produced as oxygen is in the related anodic reaction. The production of twice the “scrubbing bubbles” may be effective at cleaning the surface, except for the concern related to hydrogen embrittlement of a ferrous substrate. Thus, electrocleaning of ferrous metals are generally done in an alkaline environment in the anodic mode. Cathodic electocleaning is often left for nonferrous materials only.
In some cases, you may be able to take advantage of both types of cleaning methods by using a technique called periodic reverse cleaning. In this method, which is thought to be the most effective at oxide (rust) removal, you switch between types of cleaning (cathodic and anodic). In this combined method, it is common to finish with anodic electrocleaning to minimize deposition of loose metallic smut.
Electrocleaners are formulated with low-foaming surfactants to keep foam generation at a minimum and lower the surface tension sufficiently to enable the bubbles to release from the surface. While the cleaner is formulated with surfactants, it is generally advisable to remove heavy oils or greases with a preliminary alkaline or solvent cleaner so the electrocleaning can be most effective on oxides, smut and similar debris.