In Part I, we discussed “What is clean?” and the harmful effects of unclean. In Part II, we will describe a simple suggestion to determine cleanliness that does not require or demand (as in the case of cleanliness specifications dictated by some processes) expensive laboratory equipment, clean rooms, etc.
Pay Attention to the Water Break Free Surface
Generally, the first and simplest sign we look for in a “clean” surface is a “water break free” part emerging from the rinse steps after the soak and electrocleaner. A water break free surface should have a coating of water, adhering across the entire surface. Typically, soil left on the surface — oil or parafins in most cases, will cause water to “break” or aggressively surround an area or spot on the surface the part, exposing a bare surface without any water on the spot or break area. The surface isn’t “bare” at all; a majority of the time this means there is a contaminate covering the surface at the break area. This can be easily observed by submerging a part, before processing, in a bucket of water. Observe the water on the surface of the part as you remove the part from the bucket. You may observe water breaking, beading, and, in extreme cases, even see oil glistening on the surface of the part or an oil slick on the water in the bucket. In the words of metal finishing expert, Bill Engvall, “Here’s your sign”. I’ve been told the part finishes just fine with water break present, with no adhesion or appearance issues at all. Maybe so, BUT what longterm harm is this doing to the finish tank? Soil accumulation in process tanks can lead to conductivity inefficiencies, increase in proprietary additive usage, increased anode bag consumption, and other costly effects. This will cost you thousands of dollars per year if left unnoticed and unaddressed.
Determining Cleanliness
It is important to note some surfaces require a sufficient and sometimes robust electrocleaning step to become water break free and effectively remove surface contamination. You can see this without doing a CSI-Las Vegas style examination on what contaminates remain on the surface of parts post-soak cleaning. A simple lab project can typically determine if the contaminate removal may need addressed in the electrocleaning step by observing if there is presence of water break after each cleaning step. Also, do not confuse surface drying with water break, however, surfaces drying can also cause finish issues. Remember our definition of clean states “contaminate”, not just specifically an oil or oxide, but the generic term “contaminate”. The contaminate can come from the cleaning tank itself. Dried-on caustic can be a contaminate, leaving patterns in the finish, causing quality issues (here it is important to consider cleaning temperatures, exit rinses, seasonal changes, amongst other parameters in the pretreatment processes). In addition, accumulation of the soil removed in the soak cleaner has been seen to contaminate the electrocleaner if not properly removed by rinsing, skimming, or solution recirculation.
It is also important to note after acid pickling, the surface may give the appearance of water break after rinsing. This is not the water break we are referring to in earlier process steps; this water break is more of a shedding of water from a “clean” surface, not the repelling of water from a contaminated surface – think bucket test of water observations versus this shedding effect – very different looking when you compare the two. A more detailed study of this phenomenon should be conducted to determine if it is affecting your process. Perhaps the use of acid wetter, better rinsing, or other in line adjustments prior to finishing will produce a cleaner surface.
As mentioned above there are several other advanced methods of determining cleanliness. However, we find the most practical and telling sign in a majority of the finishing operations will be observing, interpreting, and knowing the signs of a water break free surface for your specific finishing operation. Too many times have I seen plant managers, plating operators, and executives disregard this simple sign of contamination, and it take thousands of dollars of painful testing, reworks, and tank dumps that could have been avoided, had they just read the sign.