19. The Galvanic (electrochemical) Series

All conductive elements (all metals) have different electrical potentials. These electrical potentials put each metal in a hierarchy of activity, with the most active metals at the top of the lists, and the least active at the bottom. This order of electrochemical activity is called the Galvanic Series.

A metal higher in the Galvanic Series will corrode preferentially to a metal below it in the Series. The greater the distance apart the metals are in the Galvanic Series, the higher the current that will flow between them if they are connected in the presence of an electrolyte (a conducting solution; usually water-containing dissolved salts).

Some metals like aluminium and zinc develop tough oxide films. These films give them exceptionally good corrosion resistance, although they are among the most active metals.

The position of zinc on the Galvanic Series, above most other metals, means that it will corrode preferentially if it contacts any of these metals and moisture is present. This characteristic of zinc is an important part of its exceptional performance in protecting steel from corrosion.

Many steel products are galvanized using continuous galvanizing processes. These semifabricated steel items (columns, beams, angles etc.) are subject to further processes like slitting, cutting, drilling, punching and welding.

This leaves the steel uncoated on cut edges and other areas damaged by processing. The galvanic protection provided by the adjacent zinc coating provides these steel products with their anticorrosion performance, otherwise rapid corrosion would occur on these exposed areas.

A counter example occurs with chrome-plated steel. Chromium is below steel in the Galvanic Series. When a chrome plating is breached to expose the steel substrate, rapid corrosion of the steel will occur as the presence of chromium will accelerate the rate of steel corrosion. In addition, copper plating is often used in the decorative chrome plating process and copper is also highly cathodic to steel.

Reference to the Galvanic Series is important when selecting metals that will be in contact. Their relative positions in the Galvanic Series will determine whether corrosion occur at the point of contact. For this reason copper should never be allowed in contact with steel, zinc or aluminium, nor should run-off from copper cladding be allowed to contact these other metals, as quite serious galvanic corrosion may result.

Similar care must be taken with stainless steel in contact with either mild steel or galvanized or Zincalume™ coated steel.

An often-overlooked material in the Galvanic Series is graphite – the only non-metal listed. It is lowest in the series, and thus can cause serious electrochemical corrosion problems with a range of other metals. Some rubber sealing products have high graphite content and can cause electrochemical corrosion problems.

Earlier versions of self drilling roofing screw washers were made from this material and resulted in the premature failure of the coated steel roofing and cladding on which they were used. The effect of graphite can also be observed on galvanized or Zincalume™ roofs near wood or oil fire chimney outlets. The carbon (graphite) deposits falling on the roof will accelerate the corrosion of the roofing.

The Galvanic Series diagram shows the differences in electrical potential between materials. The further apart they are in the Series, the greater the potential and the greater the corrosion current that can operate between them.

Table Voltage vs Std.Hydrogen