Zinc-rich organic coatings are known for their excellent protection against corrosion. However, their improvements remain one of the most challenging tasks in the coatings industry due to industrial standards restrictions and difficulty to find suitable zinc alternative pigments. In addition to these, ZRC industry showed increased consumption of Zn which results in increased Zn price and environmental concerns over zinc pollution, especially in marine environments. Developments of ZRC are concerned mainly in replacing part of zinc by other pigments such as carbon forms and metal alloys. The work in this thesis aims to understand the mechanism of replacing zinc with aluminium and graphite and explore the possibility of commercialising the new modified coatings. Motivated by the recent ISO12944 standard revision, this research has concentrated on exploring the possibility of replacing Zn with aluminum and graphite. Two groups of modified ZRC have been made, one with Al and the other with graphite. The anti-corrosion protection of the new coatings was tested by electrochemical tests such as EIS and polarisation and standard tests such as salt spray test and they were analysed and characterised by SEM and XRD. Results showed an improved protection of Zn-Al coatings compared to standard ZRC. On the other hand, no such improvements were observed for Zn-C coatings. A 10-15 vol% replacement of Zn by Al was found to enhance the protection against corrosion in all tests. These improvements can be attributed to the successful dispersion of Al flakes between Zn particles which increases the structure compactness and increased the galvanic action over longer immersion time. An increase in corrosion products amounts was also found in modified Zn-Al coatings with simonkolleite and layered double hydroxides as the main corrosion products in which their production was facilitated by the presence of small amounts of Al particles. SEM images showed improved barrier protection by corrosion products in the coating-steel substrate interface. Unlike Zn-Al coatings, Zn-C coatings showed lower protection compared to standard ZRC due to the presence of cathodic graphite. It was found that an addition of 30 vol% of graphite could severely facilitate galvanic corrosion at the coating-steel interface. However, an addition of 10 vol% graphite showed little improvements in scribed EIS test. Nonetheless, standard salt spray test and intact electrochemical tests showed faster failure of modified Zn-C coatings. Although graphite could enhance the electrical percolation between Zn particles, a microgalvanic corrosion was observed.
|Date of Award||1 Aug 2022|
- The University of Manchester
|Supervisor||Stuart Lyon (Supervisor) & Nicholas Stevens (Supervisor)|
- Corrosion protection
- Zinc-rich organic coatings