comcom
by Dave Wright
Senior Technical Representative
Texo Corporation
Send your Questions to davewrit@execpc.com
Q. Which is better, Iron or Zinc phosphate? A.G. Dallas,
TX
A. Whew, that's a loaded question! You have to define "better". Zinc
is generally regarded as better for corrosion resistance to iron.
Usually the substrate, specifications and existing equipment will
dictate which material you should or can use. Zinc phosphate requires
at least a five stage washer with a phosphate stage made of either
stainless or one of the new composites. Zinc costs more to purchase
and to operate. With Zinc you will automatically be dealing with a
regulated metal and substantially more sludge (a normal byproduct of
any phosphate operation). I have seen a number of conversions
recently from zinc to some of the newer iron formulations with no
degradation in performance at all. Let your parts and processes talk
to you, that will make a decision easy.
Q. We clean parts with a five stage washer that contains an
alkaline cleaner and iron phosphate prior to welding. The weldment is
then painted prior to assembly. Many times the parts rust well before
they are painted or even welded. What's happening? E.S. Chicago,
IL
A. One of the clues you provide is that the parts are rusting before
welding. Iron phosphates are not very good rust inhibitors, without a
topcoat. The purpose of the phosphate is to act as an anchoring point
for the topcoat. Iron phosphate can even accelerate rusting if not
topcoated immediately. If you need to use the washer for pre-cleaning
of parts, consider adding an aqueous rust inhibitor. There are a
number of good products on the market made by a variety of chemical
vendors that should be do the job. Many of them can be painted. This
might allow you to add it to your washers last rinse stage without
adding an extra operation. Most of these compounds are used in the
1-5% by volume range and usually pose no discharge problem.
Q. We really like our powdered cleaner, but it is a problem to
feed into the tank. Would a liquid be better? Any ideas on feeding a
powder? T.J. Phoenix, AZ
A. The use of cleaners in a liquid or powder form presents a number
of problems and opportunities for creative thinking. Liquids are
typically a little more costly, and easier to use. Liquids lend
themselves to automated dispensing, resulting in less operator
exposure. Consider premixing the powdered cleaner with warm water
making a concentrated liquid slurry. This will help to get all the
cleaner into solution more quickly.
Q. I am having a debate with our plant manager over rinse water
usage. We have a five stage washer with stages #2 and #4 being
overflowing fresh water rinses running at about 15 gpm each. I say we
need to maintain the rinses and he says we can reduce the flow. What
do you think? A.R. Saginaw, MI
A. I think you are both right. Rinsing is a very important element in
any pretreatment system. The rinse water must be kept "clean" enough
to sufficiently dilute the clinging film of process solution, from
the previous stage. This helps to stop chemical reactions and
minimize contamination of subsequent stages. This is not to say you
need to run water flows wide open to achieve a quality rinse. Rinsing
is a topic that I could devote a book to rather than a one paragraph
answer. But there are a number of very cost effective things you can
do to minimize water use in a spray washer. Here is what I would do
first. Get a hold of a conductivity meter that reads in Mhos. (micro
mhos, the inverse of an ohm). These devices are available at a
reasonable cost or see if you can borrow one (your chemical vendor
should have one). Test your fresh, incoming plant water. It should
typically read between 200 - 900 Mhos. Record this number, this it
the best that your rinse water can possibly read, today. Now test a
rinse tank. If it reads the same, or nearly the same as your incoming
water, back off the water flow (a little at a time) until the reading
starts to rise. Do it slowly and cautiously, watching the parts
carefully. As a rule of thumb, I would have no problem doubling your
incoming waters reading for this stage. Using techniques such as this
one, counter flowing and counter filling, you can maintain rinse
capability while maintaining peace with the plant manager.