galvanic corrosion of titanium

8/10/2019 Galvanic Corrosion of Titanium

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TITANIUM INFORMATION GROUP

GALVANIC CORROSION

Marine and offshore oil and gas applications involve a wide range of environments in which different metals have toperform together. Galvanic corrosion may occur when two metals of sufficiently different corrosive potential are coupledtogether directly or via an electrically conductive path in a working environment which provides an electrolytic pathbetween the exposed areas of the 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. Seawater and

chloride brines are efficient electrolytes and galvanic currents will throw over considerable distances. Oxygen dissolvedin sea water frequently plays a significant role in determining the rate and severity of galvanic corrosion. Unlike moststainless steels, the potential of titanium does not alter significantly with a reduction of dissolved oxygen in seawater.The cathodic reaction of (dissolved) oxygen reduction will normally control the overall corrosion reaction rate.

Hydrogen sulphide present in seawater or other aqueous electrolytes can dramatically affect the galvanic corrosion ratefor example by stimulation of the cathodic production of hydrogen. For this reason, titanium and its alloys must not becoupled to carbon steel, aluminium, zinc or active stainless steels at temperatures above 75C (167F) in sour sulphidecontaining aqueous environments. Under these conditions titanium will absorb hydrogen and this may lead ultimately tofailure by embrittlement.

GALVANIC CORROSIONIdeally, and as is now frequently the case, sea water piping systems are fabricated entirely from titanium and galvaniccorrosion is not a concern. The increasing use of t itanium 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 whichperform satisfactorily with titanium in sea water, may suffer corrosion on the product side of the couple. The relativesurface area of the noble (cathodic) metal to that of the less noble (anodic) metal is frequently the dominant control ofgalvanic corrosion rate. If the cathode area is small, and the anode is large, the damage to the less noble metal may beminimal. If the cathode area is large, and the anode area is small, the corrosion of the anodic metal may be mostsevere, including deep pitting, due to concentration of the corrosive attack. It is for this reason that it is essential to takecare when coupling titanium to a less noble metal if only that metal is coated. Any coating defects, damage orbreakdown in localised areas will immediately cause rapid attack of the less resistant metal unless cathodic or chemicalprotection is available or unless the adjoining titanium structure is also coated, thus effectively reducing the area of thecathode. In situations where the total surface area of titanium is large in relation to the adjoining base metal, it isimportant to establish what percentage of the exposed titanium will be effective in the galvanic cell. Tube in shellcondensers and heat exchangers are a particular case in point, and the implications of relative areas are now wellunderstood in terms of the effect of titanium tubes fitted in non titanium tube sheets. The rapidity and severity of damageto unprotected condenser tubeplates 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 of12 metres (40ft) for standard 25.4mm (1") diameter tubes. Galvanic Series based on potential measurements in flowingsea water at 35C

Material Steady State ElectrodePotential (vs.SCE)VoltsGraphite +0.25Zeron 100 super duplex steel - 0.01316 stainless steel passive - 0.0522%Cr duplex SAF2205 - 0.07Monel 400 - 0.08Hastelloy C - 0.08TITANIUM - 0.113% Cr stainless steel passive 0.15316 stainless steel active - 0.18Nickel Aluminium Bronze - 0.270-30 Cupro-Nickel - 0.2513% Cr stainless steel active - 0.52Carbon Steel - 0.61Aluminium - 0.79Zinc - 1.03Magnesium - 1.7Avoiding Galvanic Corrosion


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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 t itanium components through the use of non conducting gaskets and sleeved bolts;

Installation of short easily replaced heavy wall sections of the less noble metal; Chemical corrosion inhibition of the active metal.

Attention to galvanic compatibility is of paramount importance whenever new designs are considered or changes madeto the specification of metals or other materials including composites in an existing system.

There are a number of corrosion resistant alloys which possess potentials close to those of t itanium. In neutral, slightlyreducing and oxidising environments titanium may be directly coupled to alloys such as super duplex stainless steels,6Mo austenitic steel, 22% chromium duplex steel (but not in sea water), 625, C-276 and the like without fear of damageto either metal in the couple. Titanium may also be connected directly with metals and alloys (such as Type 410)stainless steels which are galvanically compatible when in their passive condition in a specific environment. Thesealloys may become activated, for example by local corrosion or pitting but the added effect of coupling to titanium issmall. The primary consideration must be to ensure that the alloys selected are appropriate and compatible for the

service environment.

COUPLING TITANIUM TO LESS CORROSION RESISTANT METALSTitanium should not be coupled directly to magnesium, and care is required when coupling to zinc, and aluminium insea water. This requirement applies in particular to systems where these metals are used as sacrificial anodes.Magnesium anodes must not be used in conjunction with titanium, their potential is too negative. The anodes are likelyto experience accelerated corrosion and, in the process, titanium may pick up hydrogen which is generated as thecathodic product ofthe corrosion reaction. Careful design of the system and positioning of zinc and aluminium anodes is necessary for theirsuccessful use in conjunction with titanium. When the adjacent titanium components are thin walled (such as heatexchanger tubing) or are heavily stressed critical parts, anodes must be selected or regulated to produce negativepotentials of less than -0.85v SCE. Aluminium and zinc sacrificial anodes delivering more negative potentials mayhowever be used when the adjoining titanium parts are under low levels of stress and are of heavier section. e.g. 6mm(.24 inch) wall thickness or more. A review of the cathodic protection system is essential when a significant area oftitaniumreplaces steel in a system operating in a corrosive environment. In less aggressive conditions, such as sea water,titanium may be coupled to copper based alloys and carbon steel, but effective protection such as chemical inhibition orby sacrificial anodes or impressed potential must be provided for these less noble metals. Galvanic corrosion of lessresistant metals may be harmful to titanium as the cathode if conditions lead to the uptake of hydrogen.

COUPLING TITANIUM TO MORE CORROSION RESISTANT METALSMetals and materials such as graphite and carbon fibre composites which are even more corrosion resistant thantitanium may be coupled to it and by raising the corrosion potential into the passive region will maintain the resistance oftitanium in reducing as well as neutral and oxidising environments. This protection may not be provided in the few caseswhere in very strong reducing acid conditions (e.g. in concentrated hydrochloric acid and hydrofluoric acid) where theoxide film on titanium is attacked and cannot be maintained or restored.

Chlorination

Biofilming in untreated seawater may alter the galvanic potential between coupled metals. The cathodic potential oftitanium can be increased by up to 300mV. Regular or shock chlorination, used to control biofouling will also affect tovarying extents the electrode potential of titanium and most passive corrosion resistant metals and alloys. Thepotentials of metals in a couple may switch over, or be widened to the point where galvanic corrosion between anormally compatible couple becomes possible especially at higher temperatures, or because of variations of chlorideconcentration within the system or chloride access to the cathodic areas. Potential measurements (vs Ag/AgCl) invarious oil and gas industry environments


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Metal or Sweet Sour Heavy Brine AcidisingAlloy Well Brine 1 well Brine 2 Packer Fluid 3 Fluid 4TITANIUM* 0.11 0.28 0.09 + 0.372205 0.17 0.30 0.17 718 0.16 0.29 0.19 + 0.14130 0.17 0.37 0.29 0.21

9 Cr 0.26 0.41 0.29 0.20

*Titanium Alloy ASTM Grade 19

1. 5% NaCl with 80bara CO2at 150C

2. 25% NaCl with .07bar H2S and 80 bar CO2at 200C

3. Uninhibited ,deaerated CaCl 2 brine 1200g/l , 26 bar CO2

4. Inhibited 15% HCl at 120C,

COUPLING TITANIUM TOGALVANICALLY COMPATIBLE METALSCAUTION - Crevice Corrosion Care must be taken in systems which include alloys of substantially lower resistance tocrevice corrosion than titanium. These may survive without problems in an uncoupled situation but may be susceptibleto activation by pitt ing or crevice corrosion when coupled to titanium. For example 22% chromium duplex steel may be

coupled to titanium in mildly sour oil and gas brines, but not in sea water, unless cathodic protection is provided toovercome the susceptibility of the steel to crevice corrosion.

ALWAYS CHECK COMPATIBILITY IN THE WORKING ENVIRONMENT.

galvanic corrosion of titanium

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