Titanium corrosion potentials

The results indicate when titanium is in the active state and corrodes at /corr.Ti = 0.162 A/ cm. Titanium passivates at /pass= 10 A/cm at a critical pasivating potential of Ecrit=-0. 73 V vs. SHE when coupled with Pt. The critical passivation potential is more negative than the corrosion potential of the couple E(-on ji-Pt= — 0.14 V vs. SHE. The corrosion rate of the couple decreases to 0.01 A/cm from 0.162 A/cm observed at the titanium corrosion potential, E on- xi=— 1.009 V due to Ti passivation at —0.73 V when coupled with Pt. 

Titanium has potential use in desalination plants for converting sea water into fresh water. The metal has excellent resistance to sea water and is used for propeller shafts, rigging, and other parts of ships exposed to salt water. A titanium anode coated with platinum has been used to provide cathodic protection from corrosion by salt water. 

Virtually all metallurgies can be attacked by corrosive bacteria. Cases of titanium corrosion are, however, rare. Copper alloys are not immune to bacterial attack however, corrosion morphologies on copper alloys are not well defined. Tubercles on carbon steel and common cast irons sometimes contain sulfate-reducing and acid-producing bacteria. Potentially corrosive anaerobic bacteria are often present beneath... 

The successful clinical use of titanium and cobalt-chromium alloy combinations has been reported Lucas etal. also investigated this combination using electrochemical studies based on mixed potential and protection potential theories. Verification of these studies was made by direct coupling experiments. The electrochemical studies predicted coupled corrosion potentials of -0.22 V and low coupled corrosion rates of 0.02 ft A/cm. Direct coupling experiments verified these results. The cobalt-titanium interfaces on the implants were macroscopically examined and no instances of extensive corrosion were found. Overall, the in-vitro corrosion studies and the examination of retrieved prostheses predicted no exaggerated in-vivo corrosion due to the coupling of these cobalt and titanium alloys. 

Furthermore, as we saw in a foregoing section, photoexcitation produces in a semiconductor electrode electron-hole pairs and introduces a photo-potential, which reduces the space charge potential in the semiconductor. With an n-type semiconductor in contact with a corroding metal, photoexcitation raises the Fermi level up to the flat band level of the semiconductor, thus shifting the corrosion potential in the less positive direction toward the flat band potential of the n-type oxide as shown in Figure 22.35c. Photoexcitation therefore will shift the corrosion potential in the less positive (more cathodic) direction and the corrosion will then be suppressed. With some n-type oxides such as titanium oxide, photoexcitation brings the interfacial quasi-Fermi level, peF, down to a level lower than the Fermi level, F(redox> of the oxygen electrode reaction ... 

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

The corrosion rate and corrosion potential are estimated using electrochemical kinetic parameters such as exchange current density for hydrogen evolution reaction on titanium and platinum, reversible potentials, and cathodic and anodic slopes. 

In summary, when a metal with a more negative corrosion potential (such as Ti) is galvanically coupled with a more positive metal (such as Pt), the corrosion rate of the more negative metal is accelerated. However, the anomalous behavior observed in Fig. 6.9 is explained by titanium passivation in the absence of oxidizers at more active critical potentials (negative) than the reversible hydrogen potential. 

Table 6.2 The Corrosion Current and the Corrosion Potential of Titanium Before and After Coupling With Platinum...

This means that reducing environments are compatible with relatively noble metals or alloys (copper, lead, niekel and alloys based upon these metals). When metallie materials are to be used in oxidizing environment, on the other hand, their corrosion resistance must be based upon passivity (e.g. titanium and alloys that contain sufiRcient amounts of chromium). These relationships are easy to understand when the Pourbaix diagrams for metals such as Cu and Ti (Figure 10.1) are considered, and it is kept in mind that reducing environments lower the corrosion potential and oxidizing environments lift it. Irrespective of the mentioned rule, a metal is usually most corrosion resistant when it contains the smallest possible amounts of impurities. Some natural combinations of environment and material are listed in Table 10.1. 

Examples of metals that are passive under Definition 1, on the other hand, include chromium, nickel, molybdenum, titanium, zirconium, the stainless steels, 70%Ni-30% Cu alloys (Monel), and several other metals and alloys. Also included are metals that become passive in passivator solutions, such as iron in dissolved chromates. Metals and alloys in this category show a marked tendency to polarize anodicaUy. Pronounced anodic polarization reduces observed reaction rates, so that metals passive under Definition 1 usually conform as well to Definition 2 based on low corrosion rates. The corrosion potentials of metals passive by Definition 1 approach the open-circuit cathode potentials (e.g., the oxygen electrode) hence, as components of galvanic cells, they exhibit potentials near those of the noble metals. 

For austenitic steels that are resistant to transformation on cold working (e.g., type 310), nitrogen is the element largely responsible for stress-cracking susceptibility, whereas additions of carbon decrease susceptibility (Fig. 19.10) [60]. The effect is related to alloy imperfection structure rather than to any shift of either critical or corrosion potential [59]. Stabilizing additions effective in preventing intergranular corrosion, such as titanium or columbium, have no... 

RG. 7—Spontaneous passivation of an active-passive metal, such as titanium, by galvanically coupling to a noble metal such as platinum. The noble metal has a high rale constant for the proton-hydrogen reaction thus, the corrosion potential of the system Is near to the reversible potential for this reaction [7]. 

Polarization curves are shown for four titanium materials in boiling 1 Msodium chioride with 1 iWhydrochioric add. The corrosion potentials of both 71-0.15 Pd and PdO/nOj-Ti, where no anodic peaks occur, are more noble than those of CP titanium and Ti-0.3Mo-0.8Ni. Source B. Satoh eted., The Crevice Corrosion Resistance of Some Titanium Materials, Plat. Met. Rev., Vol 31,1987, p 115-121... 

The corrosion potential of titanium vmder normally passive conditions is quite noble, but similar to stainless steel or nickel-base alloys in the passive condition. The small potential differences between these passive engineeiing alloys generally mean negligible galvanic interactions and good galvanic compatibility as long as passive conditions prevail for the alloys involved. 

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