Atmospheric corrosion measurement

The simplest form of direct atmospheric corrosion measurement is by coupon exposures. Subsequent to their exposure, coupons can be subjected to weight-loss measurements, pit density and depth measurements and to other types of examination. The main drawback associated with conventional coupon measurements is that extremely long exposure times are usually required to obtain meaningful data, even on a relative scale. It is not uncommon for such programs to run for 20 years or longer. 

The quartz crystal microbalance (QCM) is an example of a piezoelectric crystal whose frequency response to mass changes can be used for atmospheric corrosion measurements. In this technique, a metallic corrosion sensor element is bonded to the quartz sample. Mass gains associated with corrosion product buildup induce a decrease in resonance frequency. A characteristic feature of the QCM is exceptional sensitivity to mass changes, with a mass resolution of aroimd 10 ng/cm. The classification of indoor corrosivity, based on the approach of the Instrument Society of America (ISA) S71.01-1985 standard and the use of a copper sensing element and QCM technology, is presented in Table 2.7. 

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. 

In a survey of atmospheric corrosion in the Canadian arctic and sub-arctic regions rates-as low as 2-5 uva/y were recorded at inland sites . Within 1 km of the sea, rates of 21-34 m/y were measured. 

Guide for estimating the atmospheric corrosion resistance of low-alloy steels Practice for calculation of corrosion rates and related information from electrochemical measurements... 

Equipment at high or low temperatures is insulated to conserve energy, to keep process conditions from fluctuating with ambient conditions, and to protect personnel who have occasion to approach the equipment. A measure of protection of the equipment metal against atmospheric corrosion also may be a benefit. Application of insulation is a skilled trade. Its cost runs to 8-9% of purchased equipment cost. 

ASTM G84, Standard Practice for Measurement of Time-of-Wetness on Surfaces Exposed to Wetting Conditions as in Atmospheric Corrosion Testing, West Conshohoken, PA, 1993. 

Atmospheric Corrosion. The aluminum-based alloys in general are corrosion resistant to outdoor exposure with the exception of copper-bearing alloys. The Alclad alloys gave the best performance. The loss in tensile strength has been used as a measure of corrosivity and the loss of 1-2% of tensile strength over a period of 1 yr and in particular a loss of 17% was observed with 2017T alloy in 1 yr of outdoor exposure. 

Abstract Quality control of corrosion test results implies the validation of the corrosion test method and estimation of the uncertainty of corrosion rate measurement. The corrosion test in an artificial atmosphere of the salt spray mist needs evaluation of corrosivity of the test cabinet by reference specimens. Such calibration of corrosion environment raises very strict requirements for the method description and details of all procedures and used specimens. Reliable corrosion measurements by spray tests require validation of the experimental device together with the experimental procedure and determination of corrosivity uncertainty of the test cabinet environment. 

The influence of mainly SO on the corrosion rate of several materials has been shown in numerous national exposure programs. During the last decades a number of empirical relations have been derived from measurements of atmospheric corrosion rates of the most important structural metals and from measurements of environmental factors. The results are usually presented in form of equations including pollution and meteorological parameters (5.). 

However, considering the atmospheric boundary layer profiles as discussed above, the local velocity at the rack may be considerably lower, especially for test sites in either forested or urban areas. This result emphasizes the need to measure local wind speeds at atmospheric corrosion test sites, at the test rack height. Turbulence intensity measurements might be useful as well. 

Corrosion Response. The responses of metals to this test have been measured by several methods. A brief survey which follows forms a self-consistent basis of some generalizations on atmospheric corrosion. Unless stated otherwise, the examples will be of the corrosion of an alloy Ni8oFe20 ... 

NiFe)S0jj 6H20 by xray diffraction. The two types of measurement can be compared by using published density and refractive index. We believe that the discrepancy between the two may be an error in the ellipsometry due to the serious roughening of the surface. In our experience atmospheric corrosion almost always leads to sufficient roughening to affect ellipsometric data. This roughening must be taken into account in any model of the corrosion process. 

The definition of the powder surfaces tends to be in terms of the process for making it (23). This limits interpretation of their properties and also limits generalization of properties to surfaces which have been generated by atmospheric corrosion. Conversely, the LEED, Auger electron spectroscopy, photo emission properties, etc, are measured on fresh surfaces in UHV, because interpretation is difficult for more complex configurations and because experience has shown that these properties are profoundly modified and confused by exposure to real atmospheres. 

The experiment described below is an attempt to bridge this gap. Surfaces relevant to atmospheric corrosion were prepared from clean metal surfaces by reaction with water vapor under clean conditions. The properties measured — water adsorption characteristics — were selected because they were believed to have a major and direct application to atmospheric corrosion. 

Water Adsorption Experiments. The objectives were to measure water adsorption isotherms on surfaces which were well defined in terms of their preparation and also relevant to atmospheric corrosion. 

ISO CORRAG [23], the International Organization for Standardization (ISO) has implemented a classification system for evaluating atmospheric corrosivity and on the basis of variables that are fairly easy to obtain [24, 25]. This ISO classification has found several applications, for example, to predict the long-term corrosion behavior in different environments and to evaluate the effect of protective coatings. It contains two principally different approaches of assessing the corrosivity of any outdoor atmospheric environment. The first is based on exposure of standard specimens of steel, copper, zinc, and aluminum for one year whereby the corrosion effect is measured through mass loss measurements. One of five measured corrosivity classes... 

Atmospheric corrosion studies were performed in the Spanish Canary Islands exposing zinc, copper, and carbon steel sheets to the subtropical and coastal environment for a period of 3 years [45-47]. The first part of this study was to determine the corrosion rates on each sample at different times. The study also performed an analysis of the atmospheric conditions at the thirty-five test sites on the islands. With this information, the atmospheres were classified according to ISO. Measured corrosion rates were compared with the expected ISO values. The corrosion rates obtained based on ISO were determined deficient throughout the studied region... 

The relative humidity of an environment is a significant factor in corrosion of metals under atmospheric conditions. A thin layer film is formed on metals at a critical value of relative humidity depending on the nature of the metal being corroded. Dehri and Erbil [49] studied the effect of relative humidity on the atmospheric corrosion of defective polyester-coated galvanized mild steel using impedance spectroscopy. The measurements were taken at relative humidity values between 70% and 100%. The relative humidity of the atmosphere increases the corrosion rate of the underlying metal. Figure 10.11 shows the equivalent resistance circuit used to evaluate the corrosion parameters [49]. 

Experimental measurements indicated that the change in the thickness of the electrolyte affects the mass transport of oxygen, hydration ofdissolved metal ions, and accumulation of corrosion products [1,54—56]. Dubuisson et al. [57] investigated the atmospheric corrosion of galvanized steel in a micrometric electrolytic droplet containing sulfate and chloride. The measurements were performed in an electrochemical microcell through controlled... 

M. Stratmann, H. Streckel, K.T. Kim, S. Crockett, On the atmospheric corrosion of metals which are covered with thin electrolyte layers—III. The measurement of polarization curves on metal surfaces which are covered by thin electrolyte layers, Corros. Sci. 6-7 (1990) 715—734. 

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