comcom
by Dave Wright
Senior Technical Representative
Texo Corporation
Send your Questions to davewrit@execpc.com
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!