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The spread of disbondment or corrosion from a scribe or holiday in a coating film, for which the terms rust creepage or undercutting are used in this paper is an important mechanism of coating degradation. The mechanism of rust creepage has been well studied by several authors who concur that the mechanism is driven by electrochemical reactions15. The reactions occurring at the discontinuity in the coating (scribe or holiday) involve an anodic reaction in which iron is dissolved. Adjacent to the anodic region, under the coating, a cathodic reaction occurs in which oxygen is reduced to hydroxyl ions.
Rust creepage or undercutting from a discontinuity in a coating film, such as scribe or holiday, is an important mode of coating degradation on steel substrates. The mechanism of rust creepage is attributed to cathodic disbondment. This paper describes an experimental study of the influence of steel substrate topography, measured with a digital replica tape reader, on rust creepage. For the two coating systems that were studied, it was found that rust creepage was strongly inversely correlated with substrate peak density.
Modern design, manufacturing and field-testing specifications include surface profile and adhesion testing on the assumption that they are linked to long term corrosion prevention. There are a number of careful studies, dating back decades, that find no link between measurements of adhesion and (undercutting) corrosion performance, but other studies do, and the concept remains intuitively appealing and widely assumed.
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This paper will evaluate methods used to measure surface profiles created by power tools, namely air needle scalers, bristle cleaners, and roto peen scalers. Particular focus will be placed on the use of replica tape and its ability to characterize profiles of various surfaces using a digital replica tape reader with 3D imaging capability. More specifically, this paper will determine if the measurement methods described in ASTM D44171are appropriate for the measurement of profiles produced by power tools.
Coating performance and longevity is highly dependent on the quality of substrate surface preparation. The effect of profile height, profile type (e.g., type of tool used to perform the prep, angularity of profile, etc.), extent of cleanliness, and amount of chloride contamination on coating performance were all studied to determine the correlation between these factors in an attempt to identify the primary factor in coating failure due to improper surface preparation.