the newsletter of the Chemical Coaters Association International.
Reprinted with the kind permission of
By Dave Wright [deceased]
- Mequon, Wisconsin
With sadness we note Dave's passing on Oct. 11, 2013. His longtime friend Anne Goyer offers a tribute to him in The Finishing Touch, Vol. 23 No. 4.
Excerpts from 'The Finishing Touch',
the newsletter of the Chemical Coaters Association International.
Reprinted with the kind permission of
by Dave Wright
Senior Technical Representative
Texo Corporation
Q. We have a three stage washer that uses a detergent iron phosphate. We recently started to run a lot of aluminum parts. Our chemistry technical representative has suggested that we switch to a
"multi-metal" iron phosphate that contains fluoride, and that we control it with some sort of new meter. What does the fluoride do to aluminum? B.R. (from somewhere in Cyberspace via the Internet)
A. Fluoride as a component of etchants for aluminum is very common chemistry. The function of the fluoride is very similar to one of the functions of a phosphate in a conversion coating. It serves to pre-react the surface into a very stable form which resists undesirable reactions. In a phosphate conversion coating, the metal-to-phosphate chemical bonds are both strong and stable, serving to block unwanted reactions with the metal, like oxidation. In the case of aluminum in an acid solution, the surface forms a strong, stable film of aluminum fluoride. This film stabilizes the surface as the oxide is etched away by whatever acid happens to be in the solution, usually phosphoric. If the surface is not stabilized a black "smut" rapidly appears on the surface. The components of the smut are undesirable reaction products, usually salts of the aluminum, along with salts of its most common alloying elements: manganese, titanium, etc. Control of the fluoride ion concentration is necessary to gain the most efficient etching. Generally 100 to 400 PPM of fluoride is needed. The fluoride can be incorporated in to the acid solution used for etching, or as an additive. Measurement of concentration is done using a fluoride-ion-selective electrode and a pH meter (like an Orion 720A mv/pH). Once the meter is standardized against known fluoride standard solutions, measurement consists of buffering the sample by adding an excess of a buffering reagent, mixing, and dipping in the electrode and reading the meter.
Q. Which method of water purification is better for paint pretreatment Deionized (D.I.) or Reverse Osmosis (R.O.)? S.S. Atlanta, Ga
A. For most pre-paint applications, either will work. We usually look at anything under 50-60 mhos. as capable of being spot free. The only D.I. only requirement I know of is for Electro-coat paint operations. In most cases it comes down to economics. D.I. water is usually "purer". It typically comes out at 0-15 mmhos. (micro mhos - the inverse of an ohm). While R.O. water is typcially seen at 15-25 mmhos. R.O. has gained favor recently. It has a number of potential advantages such as a providing more consistent quality output over time and you don't need the cost or hazard of all the chemicals for a D.I. unit. Don't be fooled into thinking it has no cost to operate, the membranes last (on average) from 3-5 years and must then be replaced. Food for thought: I know of one company that has a need for D.I water and has implemented in plant R.O. water to feed their D.I. units. This has resulted in regeneration intervals going from daily to 4-5 weeks!
Q. We have a three stage washer. It is set up with a cleaner phosphatizer in the first stage running at a pH of about 4.6. The following two stages are ambient, overflowing tap water rinses. The incoming water pH is about 7.4. The second stage holds pH pretty consistently at about 6.8, but the third stage routinely creeps up to over a pH of 8.0! Before you call me crazy, we have recalibrated our meter and even purchased a new and we still get the same results. Help! D.G. Rockford, IL
A. Don't panic! You are not crazy, although it may be region specific and I'll bet that you are on a private or small municipality well. In order to explain it we have to start way before the washer, with rain! Rain water is typically slightly acidic due mostly to Carbon Dioxide (an acidic gas). As it seeps into the ground, it dissolves some of the minerals from the ground (especially in areas of limestone). These minerals are the main constituents of "hard" water. As the water is pumped up from the wells it still contains enough of the Carbon Dioxide to buffer the pH down, in your case, to
7.4. As the water is aerated (a spray washer is a great aerator!) the carbon dioxide is "liberated" from the water, allowing it to show it's true pH, again in your case somewhere in excess of 8.0. You may not experience this in a large municipal water system as most of them aerate their water. I experienced this phenomena with a customer a few years ago. Through a design of experiments we set up, we proved that a lower pH of about 7.2 provided a substantial increase in observed salt spray resistance. We simply put an inexpensive pH controller in tied to a small chemical metering pump and fed in phosphoric acid to maintain the pH in the desired range. It worked well and used very little acid. You can prove this to yourself (and amaze small children as well) by taking a 250ml flask half full of water and adding 2-3 drops of phenolphthalein indicator. Now slowly, add one drop at a time of 0.1N Sodium Hydroxide (NaoH), just until it turns pink - use no more than you need. Take a common soda straw, hold you breath for a few seconds, and slowly bubble the exhaled air into the flask. After a couple of exhales, the solution will turn back to clear. This proves that the pH of the water can be affected by being aerated. Have fun!
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