Background There are two types of coating failure; predictable and unpredictable. All coatings are designed to provide a certain level of performance in a specified environment. If failure occurs prematurely, something has gone wrong!
If failure occurs, it is generally the result of a problem associated with the selection or application of the coating. It is possible to evaluate the chances of a coating failing prematurely using statistical methods to determine the ‘Reliability Factor’ of a particular coating system. This work was done initially by Dr. J.F.H. van Eijnsbergen in the Netherlands to develop simple statistical methods for evaluating the reliability of any applied coating.
Relative Importance ScaleDr. van Eijnsbergen developed a scale of ‘Relative Importance’ in which a factor (I) can be allocated to each and every element in the coating process. This factor is logarithmic and rates the importance of a particular element in relation to the coatings performance.
Factor Importance 8 Very high or dominating in comparison to all other parameters 4 High or dominating in comparison with all other parameters 2 Significant over other parameters 1 Equivalent value to other parameters 0.25 Of minor significance over other parameters 0.125 Of no significance in comparison to other parameters
By allocating Relative Importance Factors (IR) to the elements making up a coating system, a Total Reliability Factor (FT) can be obtained from the sum of these. The lower the FT, the higher the reliability of the coating system.
By applying Relative Importance (IR) factors to such elements as steel surface condition, weather conditions, presence of soluble salts, surface preparation and other elements to whatever level is deemed necessary for a particular analysis (there are more than 100 factors to which Relative Importance (IR) factors could be applied in relation to coatings), the particular coating system's reliability can be given some statistical significance.
To assist in logically rating performance, a Degree of Reliability (RT) factor can be produced using the formula RT = 100 / FT. The higher the Degree of Reliability (RT), the higher the level of coating reliability in relation to other coatings.
Reliability ParametersThere are six basic parameters that impact on coating reliability. These are;
1. Steel surface and shape
2. Pretreatment of steel surface
3. Coating or coating system
4. Application of coating system
5. "Youth" period of system
6. Service exposure period
Each of these parameters has its own set of parameters which is important in determining the reliability of a coating system.
Why is Hot Dip Galvanising so Reliable?Factory applied coatings will always be more reliable than field applied coatings. It is not possible to exercise the same control of an application in the field. A simple test is to observe the performance of coatings that are in service. There are very few examples of galvanised coating failure on the literally millions of items that are in service. Signposts, fence posts, guardrail, mesh fencing, trailers, transmission towers and countless other hot dip galvanised items are still rust free with the coating intact after 20, 30 or 40 years.
Look around at painted surfaces. Almost everywhere, failures are evident. Flaking paint, rust and other signs of paint failure support a major maintenance painting industry. There is no maintenance galvanising industry.
The reason field applied paint coatings are far less reliable than hot dip galvanised coatings can be shown statistically by applying the Reliability Factor analysis to each system using the parameters that are relevant to the application of the coatings.
Parameter Relative Importance (IR) Paint Galvanising Surface condition 4 1 Surface preparation 8 8 Humidity/dew point 8 0.125 Coating Composition 8 1 Applicator skill 4 0.25 Curing conditions 1 0.125 Trransport/handling 8 1 Total reliability (FT) 41 10.5 Degree of reliability (RT) 2.44 9.5
Summary This analysis illustrates that hot dip galvanised coatings are almost 400% more reliable that paint coatings in a typical application. This analysis does not do justice to the true reliability of galvanised coatings as it deals only with the application stage of the process. Once the hot dip galvanised coating is present it will never fail from application related factors as the galvanised coating will not form on other than perfectly prepared steel surfaces. Any paint can be applied to any surface to give a satisfactory initial appearance.
Paint coating reliability is heavily dependent on application and they are statistically 400% more likely to fail than galvanised coatings.
Hot dip galvanised coatings never fail because of poor application and their performance is not affected by transport and handling.