Development of corrosion tests for "Duplex" coatings
Duplex coating is a special, two layered class of
corrosion protecting coatings. They are built up by a layer of metal on top of
the metallic material to be protected, covered by a layer of organic coating.
The metal layer acts as sacrificial anode to the base metal, providing cathodic
protection. Such coatings based on zinc or related reactive alloys are widely
used to protect iron based materials in soil or water immersed applications.
It was the goal of this 2-years project to develop a testing method addressing the protection relevant parameters of such coatings. Data from such a test should allow for optimized production and development of new coating systems.
Finally, a set of three tests was developed, providing information on the various parameters contributing to the complex mechanism of corrosion protection by duplex coatings, which had to be identified first. According to the mechanism derived, porosity of the top coating plays an important role on the overall performance of the coating system. Pores allow access to the protective metal in a wide area around a defect site but must not lead to its self corrosion at the same time.
The complete test program consists of:
(i) instrumented wet/dry cycling (WDC).
Coated samples with artificial defects are subjected to wet/dry cycles in NaCl solution. Temperature and the individual potentials are registered continously. After the test, pore density, delamination and corrosion attack are assessed.
(ii) protection current measurement (PCM).
coated samples are polarized to a selected potential in NaCl solution, usually to the protection potential of the base metal. The cathodic protection current is registered continously, its decay and time integral are evaluated.
(iii) foreign cathode simulation (FCS).
In this test, a constant anodic current is applied to coated samples with artificial defects, and the evolution of the potential is registered. The electrolyte can be varied to assess the effect on water chemistry on the tendency of the protective metal to passivate.
The multi stage testing was found necessary to assess the various parameters without their mutual interference as is usually the case in single-pass tests. They provide complementary information and allow for consistency checks. During their devolpment, additional methods were included:
Electrochemical Impedance Spectroscopy (EIS)
Electron Microscopy (SEM/EDX)
Dye penetration and others.
Reports to the customer, unpublished
P. Linhardt, G. Ball, Corrosion Testing of Duplex Coatings
for Buried Ductile Iron Pipes