Crevice titanium

Figure 29 Schematic showing the important electrochemical/chemical reactions occurring inside a creviced titanium electrode and on a Ti cathode coupled to the creviced electrode through a zero resistance ammeter.

Titanium is susceptible to pitting and crevice corrosion in aqueous chloride environments. The area of susceptibiUty for several alloys is shown in Figure 7 as a function of temperature and pH. The susceptibiUty depends on pH. The susceptibiUty temperature increases paraboHcaHy from 65°C as pH is increased from 2ero. After the incorporation of noble-metal additions such as in ASTM Grades 7 or 12, crevice corrosion attack is not observed above pH 2 until ca 270°C. Noble alloying elements shift the equiUbrium potential into the passive region where a protective film is formed and maintained. 

Fig. 7. Temperature—pH limits for crevice corrosion of titanium alloys in naturally aerated sodium chloride-rich brines. The shaded areas indicate regions...

In the construction of plants, titanium with 0.2% Pd is mainly used. It can be employed with advantage in nonoxidizing acid media and also has increased resistance to pitting and crevice corrosion because of its more favorable pitting potential [40]. 

Fig. 1.51 Crevice corrosion resulting from the crevice produced between the gasket and the flange of a titanium pipe used for conveying a hot hypochlorite solution. The attacked areas are coated with a hard deposit of titanium oxides, whilst the unattacked area of metal outside...

Griess has observed crevice corrosion of titanium in hot concentrated solutions of Cl , SOj I ions, and considers that the formation of acid within the crevice is the major factor in the mechanism. He points out that at room temperature Ti(OH)3 precipitates at pH 3, and Ti(OH)4 at pH 0-7, and that at elevated temperatures and at the high concentrations of Cl ions that prevail within a crevice the activity of hydrogen ions could be even greater than that indicated by the equilibrium pH values at ambient temperatures. Alloys that remain passive in acid solutions of the same pH as that developed within a crevice should be more immune to crevice attack than pure titanium, and this appears to be the case with alloys containing 0-2% Pd, 2% Mo or 2[Pg.169]

Metals and alloys vary in their ability to resist crevice corrosion, and this applies particularly to those that rely on passivity for their resistance to corrosion. Titanium and high-nickel alloys such as the Inconels and Hastel-loys are amongst the most resistant, but even these will be attacked under highly aggressive environmental conditions. 

Syrett and Davis conducted in-vivo studies wherein they implanted crevice corrosion specimens of Co-Cr-Mo in dogs and rhesus monkeys for up to two years. Their results indicated the alloy was not susceptible to crevice corrosion. Galante and Rostoker implanted crevice-type specimens of Co-Cr-Mo and Ti-6A1-4V in the back of rabbits for 12 months. Although no evidence of severe corrosion was found in any of the specimens, several of the titanium and cobalt specimens did show signs of single pits in the crevice regions. 

Greiss, J. C., Crevice Corrosion of Titanium in Aqueous Salt Solutions Corrosion, 24, 96 (1968)... 

Resistance to crevice corrosion Titanium is more resistant to crevice corrosion than most conventional metals and alloys, particularly where differential aeration is involved, e.g. it is very resistant to crevice attack in sea water at normal temperatures. This form of corrosion becomes more severe when acidity develops in a crevice and this is more prone to occur under conditions of heat transfer . Under these circumstances, especially in the presence of halide, even titanium may suffer attack, and the metal should not be employed in strong aqueous halides at temperatures in excess of 130°C. This limiting temperature can be raised to 180°C by use of the Ti-0- 15Pd alloy " or by coating with noble metals. (See also Sections 1.4 and 1.6.)... 

Some crevice attack upon titanium can also occur in the presence of gaseous chlorine gas at temperatures below 100°C, but this is mainly confined to crevices formed between titanium and organic sealing compounds. Here again, the Ti-0- 15Pd alloy is less prone to attack. 

In an optical micrograph of a commercially available nitinol stent s surface examined prior to implantation, surface craters can readily be discerned. These large surface defects are on the order of 1 to 10 p.m and are probably formed secondary to surface heating during laser cutting. As mentioned above, these defects link the macro and micro scales because crevices promote electrochemical corrosion as well as mechanical instability, each of which is linked to the other. Once implanted, as the nitinol is stressed and bent, the region around the pits experiences tremendous, disproportionate strain. It is here that the titanium oxide layer can fracture and expose the underlying surface to corrosion (9). 

For some materials (e.g., nickel alloys), the current is a direct measure of the rate of crevice propagation. For systems such as titanium alloys, however, internal cathodic reactions are also possible, as is illustrated in Fig. 29. This figure shows schematically the important electrochemical and chemical reactions occurring within the creviced area and on the coupled counterelectrode. This system will be used to illustrate the information that can be obtained from this galvanic coupling technique and how it can then be used directly in the development of models. 

Figure 35 Schematic showing the process of hydrogen absorption into titanium under passive corrosion conditions after a period of crevice corrosion.

What is clear from these analyses is that the avoidance of crevice corrosion will delay eventual container failure significantly, irrespective of whether it occurs by wall penetration or by HIC. With this is mind, the galvanic coupling technique (along with the associated analytical methods outlined above) can be used to compare qualitatively the crevice corrosion performance of a series of titanium alloys. Figs. 36A and B compare the parameter (/c, Ec, Ep) values ob-... 

Crevice Corrosion resistance of Alpha Titanium Alloys depends on Impurity Content/Alloying Additions... 

Titanium alloys suffer crevice corrosion in hot aqueous chloride media, but the alloy containing molybdenum, 3A1-8V-6 Cr-4Zr-4 Mo has good resistance to crevice corrosion and is successfully used in hot sour well and geothermal brine... 

---