comcom
by Dave Wright
Senior Technical Representative
Texo Corporation
Send your Questions to davewrit@execpc.com
Q. For many years our iron phosphate has had a fairly uniform gray
color with periodic spots of bright blue, about 4-12" in diameter on
parts that are 4'x2' sheet metal. We have always been told this is
normal. We looked at some equipment at another plant that does
similar parts. They don't have these spots, their parts are a uniform
color throughout. Any ideas as to why we don't have the uniform
appearance? T.H. Chicago, IL
A. It sounds like you have a condition that goes by a number of
names. They are called hot spots, cats eyes, windows, etc. It occurs
at sites on the part where the phosphate doesn't react well. This can
be due to a number of reasons, but the important thing to know is
that it is usually easily corrected. The primary cause is commonly
too little acid content in the bath. Contact your pretreatment
service person and ask the question. If they don't respond or don't
know, try reducing the pH of the bath a little bit at a time with
phosphoric acid. This should solve the problem.
Q. What is an oil splitting cleaner and how do I know if I need
one? M.W. St. Paul, MN
A. In very simple terms, two types of cleaners exist. Oil splitting
or rejecting cleaners, allow free oil to float to the surface of a
cleaner bath while oil emulsifying cleaners hold the oil in
suspension. In a spray washer, the oil splitting cleaners are
generally preferable as they allow oil to accumulate on the surface
of the bath where it can later be skimmed off for disposal or
recycling. In dip operations, emulsifying cleaners have typically
been used so that you don't pull the cleaned parts back through a
layer of oil on the surface, recontaminating them. Equipment design
in a dip tank can incorporate a weir system that allows the surface
to be continuously skimmed or swept for free oil. Since the primary
reason cleaners are "dumped" is oil contamination, keeping the oil
out of solution can greatly prolong bath life. Most of the new
"closed loop" type of systems depend upon oil splitting cleaners for
oil removal.
Q. We run a five stage washer that uses a Sodium Hydroxide based
cleaner that has typically run at a pH of about 12-12.5. Since a
major cleaning and boilout of the system, the pH falls off to about
9.9-10.5, resulting in dramatically increased chemical consumption,
and a perception of loss of cleaning power. Any ideas? A.P. Memphis,
TN
A. You didn't say how you heat the tank, but I will assume it is done
by a gas firetube. Check your firetube for leaks, and they maybe
small ones. Small pinholes in the tube can cause a leak of exhaust
gases into the bath when the burner is on, yet not allow sufficient
liquid in the tube to prevent startup. These exhaust gases are acidic
in nature and can dramatically reduce pH forming a well buffered
solution around a pH of 10. You can see for yourself how dramatic
this can be. Take a jar or Erlenmeyer flask and fill it about half
way with fresh water. Add 3-4 drops of Phenolphthalein indicator and
then add one drop at a time of 0.1N Sodium Hydroxide titrating
solution until it turns a stable pink. Now the magic, take a deep
breath, hold it for a second, and exhale directly into the container,
cap it and agitate it. If nothing happens exhale into it again, cap
it and agitate. The solution will soon turn clear due solely to the
Carbon Dioxide content of your exhaust gases. With this in mind, it
is not too hard to imagine what a burner can do, leaking directly
into the tank!