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Corrosion Special Topical Papers

Protective Coatings: Performance Evaluation and Life Prediction*

Dr. Jianhai Qiu
School of Materials Engineering
Nanyang Technological University
Nanyang Avenue, Singapore 639797


 

ABSTRACT

Methods used to evaluate the performance of protective coatings and their capability to predict the service life in real environments are discussed in the context of the degradation process of a coating system.  The accelerated tests based on existing standards (ASTM) are compared with some non-standardized methods such electrochemical impedance and electrochemical noise methods.  Fairly good qualitative correlation of accelerated test results with service performance has been reported. It is noted that  these "accelerated" tests may not really yield results in an "accelerated" way as most methods require several thousands of hours of  exposure and the they are often destructive in nature. On the other hand, electrochemical impedance and and electrochemical noise methods are non-destructive and non-accelerating in nature yet they can produce quantitative or semi-quantitative results within a few days or even hours. When real-life exposure tests are used in conjunction with the non-destructive electrochemical impedance/noise methods, realistic models for life prediction of protective coatings may be developed.

Introduction

Protective coatings is probably the most widely used method for combating corrosion. Steel structures exposed to atmospheres, buried in the soil or immersed in the sea water are commonly protected with coatings either alone or in combination with cathodic protection. This broad term - "protective coatings" encompasses metallic coatings, inorganic coatings and organic coatings as shown in Table 1.

Table 1  Types of Protective Coatings

Type of Coatings Examples
Metallic coatings hot-dip galvanizing, electroplating, electroless plating, anodizing, thermal spraying or metallizing, cladding, diffusion coating
Inorganic coatings porcelain coating, glass-lining
Organic coatings paints, vanishes, lacquers and numerous other polymeric materials that readily form durable dry films

No matter how complicated the design and formulation of a coating system may seem to be, its protective properties all boil down to two basic functions:

  • Physical Barrier Function - to separate the metal from coming into contact with a corrosive environment
  • Chemical Barrier Function - to control the micro-environment at the metal/coating interface

Any factors that may influence how the above two functions work will have an influence on the protective property of a coating system (for better or for worse). Some of these factors are coating selection, surface preparation, application condition, inspection and routine maintenance. When a coating system failed prematurely, the coating's supplier, the contractor and the facility owner often do not agree with each other over the causes of the coatings failure and who should pay for cost of rectifying the problem. Sometimes lawsuits do arise from dispute of this nature. It was reported that a coating system expected to last for 25 years failed only after 12 months in service [1]!  While it is possible to compare the relative salt spray resistance of different coating systems, there is no straightforward answer to the simple question of how long the coating will last. It is hoped that this paper will bring the coatings designers/specifiers, suppliers, contractors and facility owners a step closer in the understanding of the pros and cons associated with various methods of coatings evaluation and life prediction.
 

 

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