Titanium an answer to galvanic corrosion
Titanium an answer to galvanic corrosionKeywords: Titanium, Corrosion, Galvanizing
Attention to galvanic compatibility is of paramount importance, whenever new designs are considered or changes made to the specification of metals or other materials, including composites in an existing system.
Marine and offshore oil and gas applications involve a wide range of environments in which different metals have to perform together. Galvanic corrosion may occur, when two metals of sufficiently different corrosive potential are coupled together directly or via an electrically conductive path in a working environment, which provides an electrolytic path between the exposed areas of one metal and the other. If one or other of these three principal requirements – potential difference, electrical connection,electrolytic path – is absent, galvanic corrosion will not occur. Sea-water and chloride brines are efficient electrolytes and galvanic currents will "throw"over considerable distances. Oxygen dissolved in sea-water frequently plays a significant role in determining the rate and severity of galvanic corrosion. Unlike most stainless steels, the potential of titanium does not alter significantly with a reduction of dissolved oxygen in sea-water. The cathodic reaction of (dissolved) oxygen reduction will normally control the overall corrosion reaction rate.
Hydrogen sulphide present in sea-water or other aqueous electrolytes can dramatically affect the galvanic corrosion rate, for example, by stimulation of the cathodic production of hydrogen. For this reason, titanium and its alloys must not be coupled with carbon steel, aluminium, zinc or active stainless steels at temperatures above 75°C (167°F) in sour sulphide containing aqueous environments. Under these conditions titanium will absorb hydrogen and this may lead ultimately to failure by embrittlement (Table I).
Ideally, and as is now frequently the case, sea-water piping systems are fabricated entirely from titanium and galvanic corrosion is not a concern. The increasing use of titanium to remedy basic material corrosion problems in a variety of working environments has in some instances led to further problems. Metals galvanically close to titanium, which perform satisfactorily with titanium in sea-water, may suffer corrosion on the product side of the couple.
Avoiding galvanic corrosion
Galvanic corrosion can be avoided by:
selection of compatible materials;
protection of adjoining less noble metals in the system.
Techniques include:
coating titanium adjacent to the joint to reduce the effective cathode/anode ratio;
electrical isolation of titanium components through the use of non-conducting gaskets and sleeve bolts;
installation of shorts, easily replaced heavy wall sections of the less noble metal;
chemical corrosion inhibition of the active metal.
The relative surface area of the noble (cathodic) metal to that of the less noble (anodic) metal is frequently the dominant control of galvanic corrosion rate. If the cathode area is small, and the anode is large, the damage to the less noble metal may be minimal. If the cathode area is large, and the anode area is small, the corrosion of the anodic metal may be most severe, including deep pitting, due to concentration of the corrosive attack. It is for this reason that it is essential to take care when coupling titanium to a less noble metal, if only that metal is coated. Any coating defects, damage or breakdown in localised areas will immediately cause rapid attack of the less resistant metal unless cathodic or chemical protection is available or the adjoining titanium structure is also coated, thus effectively reducing the area of the cathode.
In situations where the total surface area of titanium is large in relation to the adjoining base metal, it is important to establish what percentage of the exposed titanium will be effective in the galvanic cell. Tubes in shell condensers and heat exchangers are a particular case in point, and the implications of relative areas are now well understood in terms of the effect of titanium tubes fitted in non-titanium tube sheets. The rapidity and severity of damage to unprotected condenser tube-plates led to an objective evaluation of the effective area, which for titanium and other corrosion-resistant materials proved in most cases to be the full length of the condenser tube, and certainly in excess of 12m (40ft) for standard 25.4mm (1in) diameter tubes.
Further details available from: The Titanium Information Group. Tel:+44(0)1562 60276; Fax: +44 (0)1562 824851.
