In an aqueous environment small insoluble particles are kept in solution by physical forces. While temperature and velocity play important roles in solution stability, electrokinetic force is the biggest contributor. Most solids suspended in water possess negative charge. Because particles with similar charges repel each other, those particles with like charges choose to remain in suspension rather than clump together and settle out of suspension. Similarly, any circumstance that would result in reduced charges on the particles would allow them to gather into larger particles and eventually “floc”.
Floc or flocculation is an agglomeration of insoluble particulates in the medium in which they are suspended. In this article we will discuss the three biggest contributors to floc in a tank used in the vacuum impregnation process.
- Ionic species are introduced into solution: The introduction of ionic species into a tank results in reduced electrostatic charge on the particles making it possible for the particles to aggregate. During the impregnation process, ionic species are regularly introduced into water by the parts that are being impregnated. Parts containing rust particles or metal parts that are susceptible to rusting in an aqueous environment release small amounts of metal ion in the water.
- Lower pH: Low pH level in a tank results in flocculation of the particles. At a low pH level an increased number of positively charged species exists in the tank water. Those positive charges reduce charges on the surface of the particles and consequently allow agglomeration. Most sealants used for impregnation purposes have low pH. Each time a part goes through the impregnation process some portion of the sealant is carried into the tanks. Because the decrease in pH is relative to the sealant amount in the water, the higher the amount of sealant the greater the chance of flocking.
- Lack of agitation: Agitation produces the necessary physical force to keep the particles apart from each other and do not allow them to spend time close to each other long enough to agglomerate. When agitation is no longer taking place, i.e. during a maintenance break, the particles are able to produce bigger particles and even floc.
In order to eliminate flocking, the above situations need to be prevented. One way to do that is to replace the dirty water in the rinse tank with fresh water. However, if the water becomes dirty too often and replacing it is impractical, chemicals can be added. One such additive is a corrosion inhibitor that buffers the pH of the water at a neutral level and binds the metal ions in the system so that they are not free to cause flocking.