Oxidation of Relay Contacts


Thread Starter

Mark Riche

What is Oxidation of relay contacts and what can be done to prevent it ??


Mark Riche, P. Eng.
Electrical Design Engineer
Northwest Territories Power Corporation
4 Capital Drive
Hay River, Northwest Territories
X0E 1G2
ph (867) 874-5266
Fax (867) 874-5264

Paul Butchart


Oxidation of relay contacts is simply the buildup of corrosion on relay contacts over a period of time. It causes problems by increasing the resistance across the contacts which, depending on
the amplitude of the voltage being switched, can cause loss of signal or overheating of the contacts.

Prevention is usually done by using high quality gold or silver plated contacts, coating the contacts with a good quality silicone dielectric compound and/or periodic maintenance to inspect the contacts and remove any buildup of corrosion.

Paul Butchart
Process Control Engineer
Qualitech Steel SBQ, LLC

Helmut Meissner

Hello Mark,

we had this problem 2 years ago. the oxidation came from magnetic devices like valves and power contactors. We tested diodes, Z-diodes and
varistors to protect the contacts from the revers induction current (I do not know the correct english expression). We had to change the relay
typ to a special typ with silver contacts (not gold!) for high current, and our coustomers did not have longer the problems.

We work with Siemens and Schrack and they told us that too much wetness in the air and too high current are making this problem.

Helmut Meissner
If possible, replace the old relay with one having silver contacts. Silver oxide is a conductor.

Michael Griffin

Oxidation of relay contacts is exactly what it sounds like - an oxide coating forming on relay contacts. There are different causes,
depending upon the type of relay and the application.

For relays switching large amounts of current, the contacts commonly "burn" due to arcing. This can often be reduced by using snubbers or diodes (depending upon the type of circuit). DC loads tend to be harder on the
contacts than AC loads, as AC arcs tend to naturally self extinguish as the cycle crosses zero.
If you are switching currents from a power supply which you can modulate, it is common practice to bring the current to zero before
switching the relays.

For relays switching very small amounts of current, especially at low voltages (e.g. signals or sensors), the cause is usually just the
natural oxidation of most metalic surfaces. This is especially a problem with small currents and low voltages because they can't punch through the
oxide layer once it gets too thick.

The usual answer here is to use special relays, such as mercury wetted reed relays (very good characteristics and very long life). Gold
contacts are also available on many relays, but be sure to check the life rating as the plating may eventually wear through.
Some standard relays are available with special contacts such as bifurcated crossbar contacts (I think that is the right term) which have a very sharp contact edge (almost like two knife blades meeting), and so tend to mechanically clean themselves. These of course have a lower maximum current rating than the standard contacts, but also a very low (or no)
minimum current rating. These are sometimes available as options in small "ice cube" relays (e.g. the P&B KHU/KHAU series).

Many small relays have *minimum* current ratings as well as maximum ones because of this problem. Using the wrong type of relay contact for the application can cause problems which are difficult and time consuming to trouble shoot. I've used both mercury wetted reed relays and the bifurcated crossbar contacts in applications which required them.
Some contactors have special low current auxiliary contacts (often as add on contacts) which are also intended to address this problem. I don't remember ever seeing minimum current ratings for the main contacts on a contactor or large relay - the assumption generally is that you would never try use them to switch very small loads.

There are special solid state relays for signal switching which are intended to replace mercury wetted reed relays. I haven't used any, so I can't comment on how good they may be.

Michael Griffin
London, Ont. Canada
[email protected]
Thanks to all of you from the list, quite a nice tutorial.
When there exist (even in few parts per million) in an atosphere surrounding metal (like contact of the relay) then there is the situation of incompatibility. It may reveal only in minutes.
In doubt and probability of that occurence I have often advised designers to include an instrument air purge, incoming from bottom and relief from top , through an instrument fitting drilled to match air flow and terminated in a goose neck.

Other purging fluid may be better suit the particulars of the project.
My name is Mario Serio and I represent Lab. Fusion MGM.

The term "oxidation" originally meant a reaction in which oxygen combines chemically with another substance, but its usage has long been broadened to include any reaction in which electrons are transferred.

The answer to the problem that you are facing, is that your relay contacts are not made of the correct material matrix.

There are various types of contact from solid platinum, gold , tantalum, iridium etc, which are primarily used for extreme applications or rugged environmental conditions Example i,e; Military Satelites, Deep Ocean drilling
equipment, Submarines etc.

As well there are contact tips for heavy industrial to light industrial applications, these contact tips are normally plated from .005 micron to.0200 microns.

For more information regarding corrosion and what you can do to resolve this

Contact Gilles Marceau he is our chief engineer with plenty of experience in these matters.
TEL 1-450-370-1477 just metnion to him that I referred you his way.

Laboratory Fusion MGM specializes in Precious Metals and Superalloys. Serving industries such as SPAR Aerospace (now EMS), ABB BOMEM and others.
Companies like these come to us for exactly these types of answers and eventually request prototypes for testing.

Keep Gilles number handy he can be a great source of information for you.

We are using magnetic proximity switches to sense actuators close or open.

Switches are SPDT type rated at 4 A @ 120 VAC.

Switches are installed in a very harsh area: high vibration, high temp, dusty, humid, and risky.

Failure rate is high and repetitive.
Common failure types:
- Weak to detect a target (magnetic)
- Completely not sensing
- Any contact failure mode sometimes found loose contacts, other time sign of burn

Switches have build-up carbon deposits "OEM findings". Wetting current has solved the problem.
We definitely have very high vibration and low current.
Kindly if you have an explanation for such issue, I would like to hear from you about solutions and recommended a suitable switch vendor.


Tony Stewart

> What is Oxidation of relay contacts and what can be done to prevent it ??

For both relay and metallic switch contacts for low current level (e.g. logic) You have two choices.

1) use Au plated contacts

2) Use a film capacitor which has low ESR to "wet" the contacts. Unless the contact is specifically designed for dry contact switching (eg telephony sealed switches) you MUST use a cap to make it a "WET" circuit. This means the current during switching is high enough to burn through a partial oxidation layer to lower the contact resistance ESR using the low ESR and stored charge of a capacitor to dump the current across the contacts. The capacitor must be close to the switch. Suitable values of Film caps start at 10nF and for say 30Amp relays use 10uF tantalum or low ESR alum. This will keep the contacts clean when used enough. Normally current must be 10% of rated current to prevent oxidation. Although the transient from a low ESR cap will be much higher than this, the duration will be so short that there is not thermal rise except at the surface oxidation layer enough to burn off the ofxide.

Tony Stewart EE '75 (retired PEng)

<b>moderator's note:</b> recently we've been getting a number of replies to very old questions. Usually I don't post them, but the last couple have been interesting.

Gerald Beaudoin

I guess this is an old post but I'll throw in my 2 cents worth anyway. We had similar problems with oxidation and corrosion in cabinets where there was traces of SO2 in the air. One solution that we found helpful was to use some of the commercially available corrosion inhibitor packets inside the NEMA4X cabinet. They must be changed on regular intervals but they seemed to do the job quite nicely. Some of them come in self adhesive pucks and can be stuck onto the cabinet walls. We used these successfully for several years. We have since become a little more pro-active and try to locate any sensitive equipment in more appropriate areas like controlled atmosphere electrical rooms, away from all the nasties.

Gerald Beaudoin