Evaluation of Atmospheric Corrosion on Electroplated Zinc and Zinc-Nickel Coatings by Electrical Resistance (ER) Monitoring
By Per Møller, Technical University of Denmark
Editor's Note: This paper is a peer-reviewed and edited version of a presentation delivered at NASF Sur/Fin 2013 in Rosemont, Ill., on June 12, 2013. A printable PDF version is available by clicking HERE.
Electrical resistance (ER) probes provide a measurement of metal loss, measured at any time when a metal is exposed to the real environment. The precise electrical resistance monitoring system can evaluate the corrosion to the level of nanometers, if the conductivity is compensated for temperature and magnetic fields. With this technique, very important information about the durability of a new conversion coating for aluminum, zinc and zinc alloys exposed to unknown atmospheric conditions can be gathered. This is expected to have a major impact on a number of industrial segments, such as test cars for the automotive industry, off-shore construction or components and devices used in harsh industrial environments. The ER monitoring makes it possible to study the corrosion rate online in remote locations as a function of temperature, relative humidity and changes in the composition of the atmosphere. Different coatings of zinc and zinc/nickel, with and without different Cr+3 conversion coatings, were tested in salt spray, and the corrosion rate was recorded every five minutes. In this paper, the results are discussed and compared.
Keywords: Corrosion measurement, electrical resistance monitoring, electrodeposited zinc, electrodeposited zinc-nickel
Introduction to atmospheric corrosion
Atmospheric corrosion of surfaces such as zinc coated steel with and without conversion coatings is normally difficult to evaluate precisely. Generally, the evaluation is carried out as a simple visual inspection during the test and subjective evaluations based upon appearance according to the ASTM D610 standard (or similar standards), which evaluates the appearance as a function of the duration of the corrosion test. However, the appearance is probably not always giving information about the actual corrosion speed.
It is in general difficult to evaluate the corrosion rate of a zinc coated surface exposed to different atmospheres and temperatures. It is even more difficult to study for instance the durability of different conversion coatings on top of the zinc layer. If the test takes place as an in situ test, it is practically impossible to obtain any information about the corrosion speed.
The corrosivities of indoor atmospheres are generally considered to be quite mild when ambient humidity and other corrosive components are under control. Nevertheless, some combinations of conditions may actually cause relatively severe corrosion problems. Even in the absence of any other corrosive agent, the constant condensation of humidity on a cold metallic surface may cause an environment similar to constant immersion for which a component may not have been chosen or prepared.
Normally, equipment used under dry conditions should not suffer from these problems unless there are large temperature fluctuations that result in condensation. Condensation can be accelerated by the presence of contaminants, especially if the contaminants are hygroscopic and adsorb enough moisture to provide a liquid layer on the surface.
The corrosion rate of zinc coatings in a car treated with different conversion coatings can be hard to evaluate in situ. Factors like chloride from deicing, temperature and moisture combined with the time and location of driving will provide numerous variables to complicate the estimate of the corrosion rate.
Classical method of evaluating atmospheric corrosion
In order to accelerate coating development and provide reliable test methods capable of quantifying the performance of coatings with respect to their corrosion resistance, accelerated corrosion test methods need to be developed. Furthermore, such methods need to be so reliable that they can be integrated into the R&D process, providing not only a better, but also a more fundamental understanding of which coating parameters and formulations have a positive impact on the durability of the developed coatings.
The more accelerated and the more reliable the applied test method is, the more favorable the accelerated corrosion test will be, since it is important to keep the required testing time as short as possible. However, the more accelerated the test method is, the less representative of the real world the test method will be, since it will involve test conditions far from actual environmental conditions. Thus, the more accelerated the test will be, the more severe the corrosion process will need to be, and the more unrealistic the test method will become. This is indeed a paradox, since an accelerated test will never be identical to long-term weathering. Nevertheless, it is important to perform accelerated tests that can simulate real, naturally occurring corrosion processes as accurately as possible. One of the more important effects to take into account is that the corrosion mechanism might totally change when altering the corrosion conditions.
A classic example of this is that it is easy to believe that an accelerated corrosion test can be performed by simply increasing the concentration of one of the corrosion-accelerating species such as chlorides. However, in many cases, an increase in the chloride concentration might actually change the stability of a surface, and thereby significantly decrease the thickness of a protective layer by changing the thermodynamically stable area for the involved compounds.1
The aggressiveness of the atmospheric corrosivity has resulted in the development of so-called corrosion classes. These corrosion classes are summarized in Table 1, which outlines the five corrosion classes in detail.
Table 1 - Overview of the five corrosion classes according to the classification provided in ISO 12944 Part.