Your
true identity has been revealed. You are the superhero,
Metalworking Man! In your red cape and flashy spandex costume,
you swoop into your shop to save the day and your shop’s
reputation. Others stand by in awe as you do battle with
your archenemy, that ruthless, relentless corroder—Rust!
Ah,
but wait. I have heard that occasionally rust has gotten
the best of you. Allow me to answer a few of the most common
corrosion-prevention questions so you, Metalworking Man,
can once again hold your head high!
I
have rust occurring just minutes after I’ve machined
a part. How can I prevent it?
If
you check, you may find that your coolant concentration
isn’t strong enough. There is enough rust preventive
in fresh coolant to provide “in-process” rust
protection. Of course, this doesn’t mean you can half-machine
a part, set it on the shelf, and expect it to be rust-free
two months later. However, parts going from one operation
to the next should be able to set for a reasonable length
of time without rusting. The fact is, when your coolant
gets old, the rust preventive wears out and a stronger coolant
mix may be necessary to provide the same rust protection
you experienced when the coolant was fresh. So, if you are
getting rust, first check the coolant concentration; then,
examine how dirty your coolant has become over the months
it has been in the machine. Dirty coolant can contain metal
fines that interfere with good rust protection. Also, mineral
build-up from your water supply can contribute to rusty
parts.
If
I coat my finished parts with a rust preventive, how long
will it protect against rust?
Rust
preventives go hand-in-hand with most coolant manufacturers’
product offerings. The type of rust preventive you use is
dependent on the amount of film you want on the part and
the length of protection needed.
Water-based
rust preventives will usually leave behind a very thin and
imperceptible film. This type of rust preventive is good
for short-term or in-process rust protection. The biggest
limitation here is that water-based rust preventives are
not the best choice where humid conditions might occur.
You can expect 30 to 60 days of rust protection from water-based
rust preventives.
Solvent-based
rust preventives offer humidity protection not found in
most water-based products. Depending on the amount of solids
vs. solvent, your film thickness will vary from imperceptible
to slightly noticeable. Solvent-based rust preventives function
much like paints. The solvent evaporates leaving behind
a rust-protective coating. Three-to-six-months protection
is typical for solvent-based rust preventives.
Oil-based
rust preventives will usually leave a slight, but noticeable
film on your parts. The oil acts as a humidity barrier and
the film will often contain several types of antioxidants
and fingerprint suppressants along with other ingredients.
Oil-based rust preventives provide exceptional protection
lasting a year or more.
Now,
here’s the disclaimer: Each of the rust preventives
mentioned have limitations based on how the film is applied,
cleanliness of the parts being coated, and the environment
in which the parts are stored. Just as you wouldn’t
store parts coated with water-based rust preventive outside,
you can’t expect freshly-machined parts coated with
quality rust preventive to provide a year’s protection
if they’re stored next to an acid bath in a plating
shop!
What
precautions can I take to prevent parts from rusting either
just after machining or after even a month or more?
Sometimes
customers dip parts immediately after machining into a stronger-than-usual
coolant mix. Then, when that “dipping coolant”
starts to get dirty or it’s the end of the shift,
often the coolant is simply dumped into the sump. This might
sound like a good idea, and it does work – but –
let me explain why this isn’t such a good practice.
Let’s say the coolant is designed to run in your sump
at 5 percent to 10 percent concentration. That means that
all the ingredients are measured out by the manufacturer
to be safe for use at these concentrations. Now, if you
double that concentration to use the coolant as a rust preventive,
you are forcing your operators to stick their hands into
a potentially harmful chemical mixture.
In
addition, when misused this way, the coolant is NOT cost
effective. I have to ask, why use a coolant that contains
lubricants, emulsifiers, surfactants, and rust preventives
to do a job that a rust preventive alone will do better?
You would be money ahead dipping these parts in a water-soluble
rust preventive designed for just such an application.
What
is the best type of rust preventive for my application?
The
answer to this question comes with another question: What
is going to happen with the parts after they are coated
with rust preventive? Are the parts going into long-term
storage, or will the parts require long distance shipping?
If yes, then an oil-based rust preventive is the best choice.
Oil-based rust preventives do not always have heavy oil
dripping characteristics. These products can be relatively
dry if the coating is applied properly. There are also heavy
wax-type products available for this type of application.
If
your customer demands parts that can go directly into an
assembly operation, then you need a water-based rust preventive
that leaves an imperceptible film and usually does not require
cleaning off before assembly.
When
a thin, dry film is required in a more humid environment,
you would be wise to choose a solvent-based rust preventive.
How
should I apply the rust preventive?
The
answer to this is somewhat dependent on the size of the
parts. If possible, parts should be dipped as this ensures
the entire part is coated. After dipping and draining, only
a thin film should remain on the part and the balance of
the rust preventive will flow back into the reservoir. Most
rust preventives can also be sprayed or brushed on metal
surfaces.
How
can I remove my rust preventive when I’m ready for
parts to go into assembly?
Usually,
rust preventive suppliers will have a cleaner available
to remove the rust preventive when needed. Generally, a
mild alkaline cleaner (Industrial type “Top Job”
or “Mr. Clean”), at 1.0 percent to 2.0 percent,
will remove an oil-based or solvent-based rust preventive.
I always suggest a low concentration “water-based”
rust preventive in the rinse stage to prevent flash rusting.
Water-based rust preventive can be easily removed with a
weak solution of water-based rust preventive in hot water.
If these parts are dried quickly, flash rusting should not
occur.
As
I mentioned, each approach to rust prevention has its limitations.
I recall a customer once asked me, “Is there a rust
preventive that doesn’t leave a film, that will protect
my parts for a year, and not be affected by humidity or
salt during long ocean voyages?” “Yes,”
I said, “it’s called paint!”
As
your shop’s resident superhero, you might feel like
it’s a battle to find the perfect rust-prevention
product for every job. But here’s where your trusty
companion and faithful sidekick, your chemical supplier,
comes in. He can work with you to experiment with the best
available alternatives. I always encourage customers to
ask questions and try new things. Who knows, perhaps someday,
working together we’ll find that one super rust preventive
that will do everything—with or without the cape!
This article originally appeared in Shop Talk magazine in November of 2005. Shop Talk is published
by Magellan Publishing.
