GALVANIC CORROSION

CORROSION: GALVANIC CORROSION

Definition:

When two different metals are immersed in a corrosive solution, each will develop a corrosion potential. If the corrosion potential of the two metals is significantly different, and they are in direct contact and immersed in an electrolyte, the more noble metal will become the cathode and the more active metal will become the anode. A measurable current may flow between the anode and the cathode. The corrosion rate of the anode will be increased and the cathode decreased. The increased corrosion of the anode is called "galvanic corrosion".

Requirements for Galvanic Corrosion:

In order for galvanic corrosion to occur, three elements are required.

1) Dissimilar metals
2) Metal-to-metal contact
3) Metals in the same conduction solution (usually called an electrolyte)

If any of these elements is missing, galvanic corrosion cannot occur. If, for example, the direct contact between the two metals is prevented (plastic washer, paint film etc.) there cannot be galvanic corrosion.

The greater the corrosion potential of each metal (the more active or more noble) the greater the potential for corrosion. The "galvanic series in seawater" lists the common metals in order from the most anodic to most cathodic (noble):

Anodic

Magnesium Zinc  Galvanized Steel Aluminum Mild Steel Low Alloy Steel Cast Iron Lead Tin Muntz Metal Yellow Brass Aluminum Bronze Red Brass Copper Alloy 400 Stainless Steel (430) Stainless Steel (304) Stainless Steel (316) Silver Gold

Cathodic

The greater the separation between the two metals listed, the greater the corrosion potential. For example, if zinc (think galvanized steel) which is an active material and near the top of the list and stainless steel, a noble metal and near the bottom of the list were in direct contact and in the presence of an electrolyte (water), galvanic corrosion will probably occur.

In addition to the three elements sighted above, the relative area of each of the exposed metal surfaces is also a consideration (see Figure below). If the area of the cathode (noble metal) is very large, and the anode (active metal) is very small, the current produced is likely to be very high and the anode will corrode quickly. For example, if there is a window frame made of stainless steel and it is attached with carbon steel screws, the screws will probably corrode. If the area of the cathode (noble metal) is very small, and the anode (active metal) is very large, the current produced will be very low and the anode will corrode very slowly, if at all. If the window frame is made of carbon steel and it is attached with stainless steel screws there will be very little, if any, corrosion.

Dissimilar metal combinations should be avoided in areas where moisture is likely to accumulate and remain for long periods. In well drained exterior applications, dissimilar metals can be used together if favorable surface ratios exist, but the best solution is to electrically insulate one from the other. When painted carbon steel and stainless steel are welded together, the welded joint should be painted. Stainless steel fasteners with neoprene or other inert washers are regularly used with other metals.

For technical help, go to the SSINA KnowledgeBase and the Nickel Development Institute at www.nidi.org Stainless Steels in Architecture, Building and Construction "Guidelines for Corrosion Prevention."