Preventing Galvanic corrosion with graphite packing
A challenge with valves is that of preventing galvanic corrosion of the valve stem. If left unchecked, this corrosion may degrade the performance of the valve and ultimately result in failure. To reduce the incidence of valve stem corrosion, it is wise to use compression packing impregnated with corrosion inhibitors.
When compression packing is used in valves and stuffing box bores, the valve stems and bores may corrode where the face contacts the packing. Pitting corrosion on valve stems and stuffing box bores is actually Galvanic (pitting) corrosion that was first identified by Luigi Galvani in 1751. As explained by Galvani, “When two dissimilar metals are in contact in the presence of an electrolyte the least noble metal (anode) will sacrifice itself to the more noble metal (cathode).”
The further apart one goes between metals on the Galvanic series of metals the faster this type of corrosion takes place. By examining the Galvanic series of metals, it is apparent that stainless steel is a much less noble metal than the graphite often used in compression packing. In addition, if there is temperature involved, this can exacerbate the amount of Galvanic corrosion that occurs. Valves and stuffing boxes made from high chromium content steels tend to be more resistant to corrosion from contact with compression packing.
Causes of Galvanic corrosion
Valve stem corrosion is more likely to occur when the valve is being stored, prior to being placed into service. This is due to the compression packing supplied with the valve being saturated with water from prior hydrostatic testing. The longer the valve is stored following hydrostatic testing, the more severe the corrosion is likely to be.
Hydrostatic testing is high-pressure water test of a valve carried out by valve manufacturers to identify any weaknesses in a valve body, prior to actual use. Hydrostatic testing of valves is a troublesome situation for packing manufacturers because many valve manufacturers carry out hydrostatic testing with a graphite compression packing in place. Ideally, valve manufacturers should perform a hydrostatic test with junk packing in place and, following the test, replace this packing with the actual packing that will be supplied to the customer. This would eliminate the residual water which accumulates in the test packing and which can propagate Galvanic corrosion in the stored valve.
Preventing Galvanic corrosion when using graphite or carbon packing
To protect against the effects of Galvanic corrosion, valves and stuffing boxes should be packed with a compression packing which contains a corrosion inhibitor. The standard corrosion inhibitor is zinc, which is high on the anodic scale and sacrifices itself to protect valve stems or stuffing box bores against Galvanic corrosion arising from graphite in the packing.
Canada Rubber Group Inc. (CRGI) supplies graphite and carbon compression packing from Teadit North America which contains corrosion inhibitors. These high-quality packings provide superior sealing performance while preventing
Galvanic corrosion in valve stems and stuffing box bores. To see the full range of Teadit compression packing which we carry, please visit our compression packing page, or contact our sales department at email@example.com.