CORROSION IN BRASS INSTRUMENTS
There is a lot of
misunderstanding about corrosion in brass instruments.
We hope this article dispels the myths, helps you understand causes and
steers you towards preventing of corrosion.
Pistons and rotary
valves are made of a variety of metals: Brass, Monel, and stainless steel.
Years ago, brass was used in all piston valves.
The ease with which naked brass corrodes made it necessary to plate the
piston surface with nickel. This
worked well because Nickel resists corrosion, but, there was a drawback.
Nickel plating was not simple to apply and it can flake off.
This necessitated re-plating and re-lapping of the valves.
Even during manufacture of the horn, the plating procedure was
troublesome. The cure for this was an alloy that was corrosion resistant and
easy to manufacture; that alloy is Monel.
Monel (also called
Monell) is a patented
Nickel/Copper alloy (66/29) plus significant additions of Cobalt, Manganese,
Iron, Silicon, Carbon and Aluminum. It is named after the Canadian metallurgist
Ambrose Monell who developed it. (Mr.
Monell died in 1921.) Its
resistance to air oxidation and reaction with carbon dioxide makes it reasonably
worry-free. When corrosion does occur it
is at the boundary where the crystals of Nickel and Copper join. This
would suggest that some batches of Monel might have better resistant to
corrosion than others. Monel
is also a very hard and wear resistant alloy.
Nearly all new horns have piston valves with a Monel surface.
Do Monel valves
corrode? Yes, albeit not seriously
so. Most trumpet manufacturers deny
this, but store owners tell a different story.
I recall looking at a new Yamaha trumpet at the NAMM show one year.
I asked the salesperson if the pistons ever suffered from corrosion.
She said no, but when I lifted the piston up from the valve casing of a
brand new trumpet, and asked her what that green spot was, she became upset and
asked me to leave. At the same
trade show, another manufacturer confessed that their pistons also developed a
"blemish" during use.
The next time you
clean your trumpet, look closely at the face of the piston.
You will probably see a circular spot about the size of the adjacent
port. This is corrosion, and may
vary in color from pink to a greenish brown.
(Copper and nickel form hydroxy carbonate and chloride complexes which vary in
color from pink to green and brown.) Although disconcerting, the corrosion is not
immediately serious.
If desired, the spot can usually be removed with a mild abrasive such as
toothpaste or Bon Ami paste.
The cause of this
corrosion - unique to the Monel face of the piston - is not readily understood.
A few years ago we studied this phenomenon for the Conn corporation, and developed a theory. The Monel
piston face is close to both the brass surface of the valve casing and solder
joints. Moisture in the breath can
act an electrolyte at the point where the moisture passes through the valves.
Monel, brass and solder are dissimilar metals, and, therefore, can
promote galvanic corrosion. Our
theory explains why piston corrosion is reduced or prevented by using
valve oils which cling to the valve surface, and are slow evaporating.
The oil acts both as a dielectric and moisture repelling film.
Except for the piston surface, all the other parts of a Brass instrument are made of brass. Brass is an alloy made primarily of Copper plus zinc, and it is readily attacked by a variety of seemingly harmless substances such as spit and sweat. Corrosion of brass can be serious. It occurs where the tubing bends, on the places where the hands are usually placed, and on each end of valve slides. We routinely hear people say, " my breath is very acidic, and it really attacks the metal". We have even heard it from trumpet repair people.
No
The truth is that
there is no such thing as acidic breath. If
your breath was acidic, it would attack your teeth faster than a 24 oz Hershey
candy bar. No, your breath is very
slightly alkaline, and you can test it yourself with litmus paper. As for those that say their breath is corrosive,
we can agree; but for reasons they would not have expected.
The human breath
contains carbon dioxide, enzymes, some fugitive fatty acids, salts and a lot of
moisture. When you blow into the
horn, the moisture deposits everywhere. Most
of the water leaves via the water key, but a thin mucoidal film of moisture
laden with carbon dioxide, spit and bacteria clings to the interior walls of the
instrument. This film slowly
attacks the metal and forms the corrosion product (copper hydroxy carbonate).
This alkaline product can bridge across joints, and build up a
multicolored corroded layer. If left unchecked, the slides will become nearly impossible
to move without acid dipping.
The exterior of the
horn is protected by a coating of lacquer.
If this coating is worn off, the exterior surfaces can suffer from a
different type of corrosion. This
corrosion is the reaction of the salt and fatty acids in your fingers with the
zinc in the brass; the process is called red rot or dezincification.
As the components in your sweat attack the brass, the zinc is selectively
extracted until only a porous layer of red
colored copper is left behind. The
process is slow, and depends on the amount of sweat, and the length of time the
skin debris is in contact with the metal. If
not further protected, the metal in that area becomes weak, and is easily
perforated.
So there we have
it. The question now becomes, is it
preventable without having the horn plated ? Yes.
Rotary valves have
special considerations, and we have a separate article on them.
* Oh, the anecdote. Well, at the NAMM show one year, we learned from several store owners that our Pro-Oil Blue formula seemed to prevent Monel staining. We spoke to the Conn people about this, and offered to share some of our research with them. Naively we thought that they might decide to use our oil in their trumpet line. But instead, they quickly introduced their own valve oils: one was red colored, and the other was blue. Isn't that amazing!