Abstract
Hardfacing alloys are used extensively in mining, oil and gas, metallurgical and otherindustries, particularly those based on high chromium content and chromium carbides.
While these alloys are valued for high wear resistance and hardness, they are also used
in applications involving corrosion resistance. However, limited research has been
conducted to understand and improve the corrosion resistance of hardfacing alloys. Hard
surfaces can be produced by welding, cold spray, laser surfacing, ion implantation, etc.
In this investigation, welding and cold spray depositions of high chromium content-based
alloys were studied for corrosion resistance in artificial seawater. Within this, focus
research areas were identified. Firstly, as these alloys are used extensively in mining, they
can be affected by microbiologically influenced corrosion. The microstructure and
composition of alloys can also be affected when subjected to heat treatment and
nitrogen additions, especially as they are generally known to increase corrosion
resistance. Furthermore, thermodynamic analysis such as Pourbaix diagrams can
provide detailed information with respect to stable phases to understand the inherent
corrosion behaviour of the hardfacing alloy when nitrogen is added. Based off these focus
areas, research provided the following outcomes
Corrosion of High Chromium White Iron (HCWI) was studied under microbiological
conditions using Pseudomonas Aeruginosa. Alloys involved different compositional
content such as chromium content, as well as differing microstructural characteristics
such as austenite and martensite. Higher chromium content was associated with
increased corrosion resistance in the austenitic HCWI alloys, suggesting potential for
development in its optimisation.
Heat treatment is another method of modifying microstructure. For cold sprayed powders
consisting of agglomerated nickel chromium – chromium carbide (NiCr-CrC) particles,
heat treatment tends to increase corrosion resistance. The temperatures chosen (650°C
and 950°C) have also been known to initiate further precipitation or dissolution of
carbides. The samples at target temperatures were held for six hours in air atmosphere
and cooled in air, leading to the modification in size and number of carbides. with
treatment at 650°C producing the best corrosion resistance results. Carbide
disintegration at 950°C was attributed to the poorer corrosion results than the 650°Cheat
treated samples, although still better than the as-sprayed sample.
Similar heat treatment processes-based investigation for corrosion resistance of welddeposited
HCWI alloys at 650°C and 950°C produced different results compared to the cold sprayed samples.
In this case, heat treated HCWI samples produced secondary carbide precipitation compared to
the as-deposited state, with 650°C having more carbide precipitates than at 950°C. Heat treatment
also produced better corrosion resistance at 950°C than at 650°C, with heat treatment overall
confirming improved corrosion resistance compared to the initial as-deposited state.
Apart from heat treatment, shielding gas usage is another method that can affect
microstructure. Nitrogen was used as shielding gas for weld-deposited HCWI alloys to
study this effect. This was found to increase the nitrogen content in the matrix as well as
increased the volume fraction of the austenite matrix, with higher nitrogen gas flow rate
at 15L/min produced higher corrosion resistance.
In relation to nitrogen alloying with the expectation that nitrides would be produced,
Pourbaix diagrams were developed which reveals information about the stability regions
of the alloy under various potential and pH conditions. The electrochemical equilibrium
diagrams found showed that chromium nitrides have a larger immune region than
chromium carbides. This suggests that adding nitrogen is beneficial for the corrosion
resistance of hardfacing alloys. Results from the Pourbaix diagram can thus aid in the
selection of environment and cathodic potential required to limit corrosion of high
chromium alloys containing chromium nitrides and carbides.
By studying these different environments and microstructural modifications, methods of
improving corrosion resistance for high chromium content alloys has been investigated
in this thesis. It is expected that these methods can further help industry to provide
optimal outcomes and reduce costs for maintenance.
| Date of Award | 2025 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Supervisor | Kannoorpatti Krishnan (Supervisor), Naveen Kumar Elumalai (Supervisor) & Suresh Thennadil (Supervisor) |