Editorial Feature

Galvanizing - Reliability Factors for Hot-Dip Galvanizing

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Coating failure can be either predictable or unpredictable. All coatings are intended to offer a specific performance level in a given environment. A premature failure indicates that there is a problem.

In general, failure occurs due to a problem related to the choice or application of the coating. The likelihood of a premature coating failure can be evaluated using statistical techniques to evaluate the “Reliability Factor” of a specific coating system. Dr J.F.H. van Eijnsbergen from the Netherlands was the first person to perform this study to devise easy statistical techniques for assessing the reliability of any applied coating.

Relative Importance Scale

Dr van Eijnsbergen created a scale of “Relative Importance” where a factor (I) can be allocated to every element in the coating process. This factor, which is logarithmic, rates the significance of a specific element with respect to the performance of the coating.

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

A Total Reliability Factor (FT) can be obtained from the sum of the Relative Importance Factors (IR) by allocating them to the elements that constitute a coating system. The reliability of the coating system is inversely proportional to FT.

Some statistical significance can be given to the reliability of a specific coating system by applying IR to elements such as weather conditions, steel surface condition, surface preparation, presence of soluble salts and other elements to a level that is considered necessary for a specific analysis (IR could be applied to over 100 factors in relation to coatings).

Coating performance can be logically rated by producing a Degree of Reliability (RT) factor using the formula RT = 100 / FT. The Degree of Reliability (RT) is directly proportional to the level of coating reliability with respect to other coatings.

Reliability Parameters

Following are the six basic parameters that influence the coating reliability:

  1. Pre-treatment of steel surface
  2. Steel surface and shape
  3. Coating or coating system
  4. Application of coating system
  5. Service exposure period
  6. “Youth” period of system

Each parameter mentioned above has its own set of parameters that are crucial in establishing the reliability of a coating system.

Why is Hot-Dip Galvanizing so Reliable?

Compared to field-applied coatings, factory-applied coatings are more reliable as the same control cannot be exercised for an application in the field. A simple test would be to observe the performance of coatings that are in use. Of the millions of items that are in use there are very few examples of galvanized coating failure. Fence posts, signposts, mesh fencing, guardrail, transmission towers, trailers and several other hot-dip galvanized items remain rust free, where the coating is intact even after four decades.

While looking around at painted surfaces, failures are obviously seen nearly everywhere. Rust, flaking paint and other signs of paint failure are responsible for the sustenance of the maintenance painting industry. However, a maintenance galvanizing industry does not exist.

Reliability Factor analysis can be applied to reveal the reason behind the relative unreliability of field-applied paint coatings compared to hot-dip galvanized coatings, by assessing each system by applying the parameters that are relevant to the application of the coatings.

Parameter Relative Importance (IR)
Paint Galvanizing
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
Transport/handling 8 1
Total reliability (FT) 41 10.5
Degree of reliability (RT) 2.44 9.5

Summary

This study demonstrates that hot-dip galvanized coatings are nearly 400% more reliable compared to paint coatings in a typical application. However, it does not reveal the true reliability of galvanized coatings since it is only concerned with the application stage of the process. The hot-dip galvanized coating will never fail due to application-related factors once it is applied. This is because the galvanized coating forms only on perfectly prepared steel surfaces, whereas it is possible to apply any paint on any surface to give a satisfactory initial appearance.

The reliability of the paint coating is largely dependent on application. Moreover, statistically, paint coatings have a 400% greater chance of failure compared to galvanized coatings.

Hot-dip galvanized coatings never fail due to poor application and their performance is not influenced by handling and shipping.

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