Thin film corrosion monitoring

Figure 5.32 Corrosion sensor and access fitting used for thin film corrosion monitoring. (Courtesy of Kingston Technical Software)...

Impedance spectroscopy This technique is essentially the extension of polarization resistance measurements into low-conductivity environments, including those listed above. The technique can also be used to monitor atmospheric corrosion, corrosion under thin films of condensed liquid and the breakdown of protective paint coatings. Additionally, the method provides mechanistic data concerning the corrosion processes, which are taking place. 

Acoustic wave (AW) devices are ideally suited to thin film characterization due to their extreme sensitivity to thin film properties [10]. The sensitivity of AW devices to a variety of film properties (see Chapter 3), such as mass density, viscoelasticity and conductivity, makes them versatile characterization tools. The ability to rapidly monitor changes in device responses resulting from changes in thin film properties permits their use for monitoring dynamic processes such as film deposition, chemical modification (e.g., photo-polymerization, corrosion), and diffusion of species into and out of films. 

Using AW devices to monitor dynamic processes such as diffusion and corrosion can dramatically reduce the time required to quantify these processes. For example, as discussed in Section 4.2.2, diffusion equilibration times typically increase with the square of the diffusional length. For a thin film, this length scale, the film thickness (h), is very small. This enables the quantification of diffusion coefficients as low as 10 cm /sec in less than one day, whereas months would be required using many conventional techniques that use thick films or bulk samples. For corrosion monitoring, the dramatic decrease in mass detection limits obtainable using coated AW devices, as compared with conventional balances and sample coupons, allows detectable mass changes to be achieved in minutes or hours rather than days or months (Section 4.4.3). 

In conclusion, the results presented in this chapter demonstrate the extreme versatility of AW devices for the characterization of materials. The inherent sensitivity of AW properties to the mechanical and electrical properties of thin films can be used to advantage to directly monitor a wide variety of film properties. Since the properties and behavior of thin-film materials can be very different from those of similar bulk materials, this ability to directly measure thin film properties can be a significant advantage in materials research and development. The ability to use thin films instead of bulk samples has the added advantage that the time required to perform an evaluation of dynamic processes such as diffusion and corrosion can be greatly decreased. The number of applications of AW devices to thin-film characterization continues to increase, and is limited only by the ingenuity of AW device researchers and developers. 

Direct measurement of roughening has proved to be a sensitive monitor of the corrosion of thin films. 

Relative humidity leads to the formation of a thin surface film on a metal when exposed to rain, fog, or dew formation. Xu et al. [52] monitored the dew formation process with a specially designed experimental arrangement. The results indicated that dust on the metal surface facUitates dew formation and increases atmospheric corrosion. Atmospheric corrosion requires both a thin film as well as some type of contaminant to initiate. The process of dew formation occurs much more rapidly in the presence of salts on the metal surface. Figures 10.13 and 10.14 compare the development and advancement of the dewing process on a clean and dust-contaminated surface, respectively [52]. Dew formation was monitored on mild steel (i) before dewing (ii) after 2 min (iii) after 4 min (iv) after 8 min and (v) after surfece drying (ambient temperature 13 °C, relative humidity 69). If the d.c. current for... 

Figure 5.8 displays the Fe L-edge absorption spectra of the iron film deposited on a quartz crystal wafer before and after several hours of corrosion at relative hnmidity above 90% in a flow of atmospheric pressure of air. The quartz aystal wafer was connected to an electronic oscillator circuit monitor the small mass changes in the deposited thin film. Clearly, the Fe L absorption edge was shifted to higher energy after corrosion, consistent with the oxidation of iron due to corrosion. The quartz crystal microbalance (QCM) also revealed a mass change of approximately -h 15 ag/cm, likely due to the corrosion of the iron film into FeOOH. 

Corrosion monitoring studies of this nature have proved useful for identifying process conditions that lead to the formation of highly corrosive thin-film electrolytes, revealing the most corrosive areas, and evaluating materials designed to resist such attack in the most cost-... 

More recently, SECM-QCM has been employed to monitor localized corrosion processes [166], the electrochemical growth of poly(o-phenylenediamine) thin films [167], and ion exchange at the poly (butyl viologen)-electrolyte interface [168]. 

T. H. Muster, B. A. Sexton, F. Smith, R. O Halloran and T. Davis, Thin-film sensors for atmospheric corrosion and structure monitoring , in Proceedings of the Corrosion Control and Non-Destructive Testing Conference, November 2005, Gold Coast, Queensland. 

Studies of corrosion processes, detailed in Section 4.4.3, have demonstrated the capability of SAW devices to monitor relatively low rates of chemisorption, including the conversion of a thin copper film to CU2S at an initial rate of 4% of one molecular monolayer/day. The use of SAW devices to monitor the real-time desorption of species from a metal film in response to a temperature ramp has been shown to yield information about both the energy and extent of chemisorption [114]. TSM studies of chemisorption of O2 and CO on very thin Ti films were used to determine that the oxide being formed is Ti203 and that the oxidation depth is approximately one nm [137]. For further discussion and additional examples of chemisorption, the reader is referred to Section 5.4.4.3, where these... 

The ability to use SAW devices for real-time monitoring of corrosion of thin metal films in gaseous environments has been demonstrated. Metal films, ranging in thickness from a few Angstroms to a micron, were deposited on the prop-agatimi path between transducers. Since the acoustic-wave propagation velocity depends upon the mass and mechanical properties of the film, any change in these properties due to corrosion alters the velocity. 

In earlier studies using a fixed Kelvin probe, corrosion kinetics and mechanisms were studied without the spatial resolution possible with the SKP [222, 223]. The use of a Kelvin probe as a reference electrode in corrosion studies with very thin electrolyte films (2 pm) has been described [224]. The use of Kelvin probes to control and to monitor the potential has been reviewed [225]. 

Continuous, on-line corrosion monitoring can be used to operate FGD systems at optimal efficiency while avoiding high corrosion rates. Conventional corrosion monitors are designed for use in bulk liquids and do not function satisfactorily in the thin condensate films that form on the surfaces of FGD ducts. Recent EPRI research has made use of an advanced electrochemical method of corrosion surveillance developed by the Corrosion and Protection Centre Industrial Services (CAPCIS) in England [30]. This method allows on-line monitoring of corrosion activity in either thin condensate films or bulk liquids. EPRI introduced the CAPCIS system to the U.S. electric utility industry in 1985, sponsoring several field demonstrations. 

The number of industrial applications of electrochemical noise monitoring has grown significantly in recent years, yet skepticism about the universal usefulness of this technique is still encountered. It has been shown to be well suited to monitoring under thin moisture film conditions, such as those encountered in flue gas condensation and atmospheric corrosion. Present concerns and controversy are mainly related to the validity of corrosion rates derived from noise records. 

---