TechArchive

Original article date: April 1998

Corrosion within pneumatic circuits and its consequences – piston seizing, leakage, jamming of control valves, reduced efficiency – are due to the combined action of atmospheric and electrolytic corrosion, the latter being largely predominant in pneumatic systems.

In the electrolytic process, the water contained on the ambient relative humidity of the air plays the role of the electrolyte. The galvanic action is the result of the potential difference. In threaded connections, the three forms of electrolytic corrosion can occur in a number of combinations. In order to avoid corrosion, two general principles should be applied:

First rule: compatibility of different base metals

The susceptibility of different base metals to corrosion whilst in contact depends upon the difference between the contact potentials, or the electrolytic decomposition potential of the metals involved. The greater the potential difference, the greater the tendency for corrosion. The metal with the higher negative potential forms the anode and is corroded.

Examples:

brass on copper = very slight corrosion

brass on zinc = heavy corrosion

steel on zinc = medium corrosion

steel on copper = heavy corrosion

Non-passivated stainless steel has a decomposition voltage of about 0.70V and is attacked vigorously by copper or brass.

Passivated stainless steel has a reduced decomposition voltage of -0.24V. Passivation is obtained by the direct oxidation of the chromium compounds contained in the steel. Surface damage from scratches, solder or other contaminants will expose the base metal and reactivate the higher decomposition voltage.

Second rule: dry compressed air

Water acts as an electrolyte and a close relationship exists between the relative humidity of air and electrolytic corrosion. The mass of water vapour contained in the air is measured in weight (g) per unit volume (cu.m.) of air. The saturation temperature, or the dewpoint, is the temperature at which the relative humidity of the air is 100%. The corresponding mass of water vapour has an upper limit which is dependent upon, and increases with, temperature.

Cooling below the saturation temperature causes condensation through the formation of water droplets. In practice, electrolytic corrosion becomes significant when the relative humidity of the ambient air is in excess of 50%.

• The pneumatic connectors design manual, available free from the Brass Products Division of Parker Hannifin, contains useful advice on many aspects of the subject. It also features new and recently products, including Prestolok Micro – a miniature push-in connector – brass adaptors for welding robots, blow guns, low pressure hoses and tubing and EO light series steel fittings.

Mass of water vapour at 100% relative humidity

• Parke r Hannifin Fluid Connectors Group
• 01332 365631

April 1998