Abstract
Titanium tubes of the condensers of MSF desalination plants exhibit excellent resistance to general-, pitting- and crevice corrosion. They withstand erosion-corrosion and impingement attack of flow velocities as high as 20 m/s. They are also immune to SCC and are unaffected by polluted seawater. Titanium tubes are generally mounted on copper-base tube plates, with which they are galvanically incompatible. Severe corrosion of the tube plates occurs, and cathodic protection is commonly applied. Unless carefully controlled cathodic protection will shift the potential of tube ends towards negative values allowing the absorption of hydrogen. Hydrogen in titanium induces brittleness leading to tube failure. We have developed a simple electrochemical technique to study the mechanism of hydride formation on titanium coupons (representing Ti tubes) connected to 70Cu-30Ni samples (representing tube plates) when cathodically protected in natural Arabian Gulf water. Cathodic protection was carried out by sacrificial Fe, Zn or Mg anodes. When Ti is coupled to 70Cu-30Ni coupons, it acquires a potential very near to the corrosion potential of the copper alloy. Upon disconnection the Ti potential instantaneously returns to the original open circuit value, measured before coupling. Short circuiting the Ti/Cu-Ni couple to sacrificial anodes of Fe, Zn or Mg shifts the couple potential towards high negative values. When disconnected, the Ti potential decay curves give rise to two distinct arrests before reaching the original open circuit value. The first step (Step A) develops at the most negative potentials and is recorded after long coupling to Zn or Mg anodes. It represents the desorption of occluded hydrogen. The second step (Step B) is recorded with all three sacrificial anodes. It is well defined and its potential is independent of anode material. With one and the same anode, the length of Step B increases with the time of cathodization. It depends also on temperature and aeration of the solution. Step B is attributed to the decomposition of bulk TiH2. Microscopic examination of the coupons reveals the presence of feather-like structure characteristic of TiH2 phase. Coupling of Ti to Fe produces a new step (Step C). This step results from the deposition of Fe(OH)(2) on the metal, where it acts as weak hydrogen donor. The results of cyclic potentiodynamic polarization experiments of H-2-charged Ti electrodes support the idea of formation of two types of hydrides. Because of the continually changing potentials, the two hydrides decompose over a wide range of potentials.