Supercritical CO2 erosion protection

In summary, the use of supercritical CO2 in industrial processes and applications presents a major challenge in terms of material erosion. Stainless steel, commonly used in these processes, is susceptible to erosion in the hottest zones and areas where the fluid changes direction or expands/compresses. Some suggest using superalloys for critical parts, but an alternative solution could be coating the stainless steel with an erosion-resistant material. However, there are limitations to this approach as the surface oxidation of stainless steel makes it difficult to form a protective coating. Some potential coating options include ceramics, carbides, and iridium electroplating. Additionally, research is being conducted on modifying the surface of stainless steel to improve its corrosion resistance in supercritical CO2 environments. It
  • #1
Hi. As i understand from what i have read about supercritical CO2 loops a major hurdle is erosion of the materials. especially in the hottest zones of the loop and in thin material sections and zones where the fluid changes direction or expands/compresses. But from the experiments i have read they have usually used plain uncoated stainless steels.

And some suggest making critical parts out of superalloys. But can you not just coat the stainless steel with a erosion resistant coating in stead of making the whole part out of superalloys? And if so what sort of coating material will be suitable? Will for example titanium nitride coating, or hard chrome plating help against erosion?
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  • #2
Depending on the sCO2 application there will be other chemical reactants and contaminants in the mix that may cause corrosion, followed by erosion of those compounds.

The problem with stainless steel is due to it's surface of chromium oxide. It is only stainless in the presence of oxygen, without excess oxygen any sulfur will turn it to a black crumb.

It is difficult to coat an oxidised surface with a protective metal layer, without first removing the oxide. Hard chrome on stainless steel will simply form another stable chromium oxide like the original surface of stainless steel, so it will probably not make much difference.

Maybe you could consider ceramics, or a surface treatment with materials like carbides that are hard and do not react with any component of the soup. Take a look at the materials used for metal cutting tools.

Iridium electroplating worked well on fountain pen nibs. Maybe it will work with sCO2. The question becomes, what base metal do you employ?
  • #3
Stormer said:
And some suggest making critical parts out of superalloys. But can you not just coat the stainless steel with a erosion resistant coating in stead of making the whole part out of superalloys? And if so what sort of coating material will be suitable? Will for example titanium nitride coating, or hard chrome plating help against erosion?

Corrosion of a stainless steel and nickel-based alloys in high temperature supercritical carbon dioxide environment, V.Firouzdor, K.Sridharan, G.Cao, M.Anderson, T.R.Allen

Penetration of protective chromia scales by carbon, David J.Young, Thuan Dinh Nguyen, Peter Felfer, Jianqiang Zhang, Julie M.Cairney

Attack of the chromia layer is a problem. Atom probe analysis shows that carbon penetrates the scale via oxide grain boundaries. Attack on the grain boundaries means that grains can lift out, hence the erosion.

Corrosion behaviors of four stainless steels with similar chromium content in supercritical carbon dioxide environment at 650 °C, Hongsheng Chen, Sung Hwan Kim, Chaewon Kim, Junjie Chen, Changheui Jang

This approach may be promising.
Surface modification of austenitic stainless steel for corrosion resistance in high temperature supercritical-carbon dioxide environment, Sung Hwan Kim, Gokul Obulan Subramanian, Chaewon Kim, Changheui Jang, Keun Man Park
• Al and NiAl layers were coated on 316LN, followed by inter-diffusion treatment.
• Surface-modified specimens exhibited lower weight gains in S-CO2 at 650 °C.
• Pre-oxidation at 900 °C formed α-Al2O3, which further improved corrosion resistance.
• NiAl coating showed smaller inter-diffusion zone before and after S-CO2 exposure.

Corrosion and carburization behavior of Al-rich surface layer on Ni-base alloy in supercritical-carbon dioxide environment, Ho Jung Lee, Sung Hwan Kim, Hyunmyung Kim, Changheui Jang
• Al-rich layer was developed on Alloy 600 by Al deposition and EB remelting.
• When exposed to S-CO2 at 600 °C, mostly Cr2O3 with transition Al2O3 was formed.
• Carburized region of amorphous C layer was observed at the oxide/matrix interface.
• α-Al2O3 was formed after pre-oxidation which resulted in superior resistance.

See also this open source study, which unfortunately is thin on sCO2.
Long-Term Performance of High Temperature Alloys in Oxidizing Environments and Supercritical CO2

One also has to consider the thermodynamic stability of the metallic (mechanical) substrate, and differential thermal expansion of the coating and metallic substrate.

Corrosion of Structural Materials for Advanced Supercritical Carbon-Dioxide Brayton Cycle
Looking at 316, 310, 709 and others Reports/FY 2013/13-4900 NEUP Final Report.pdf
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  • #4
Possibly no need for supercritical CO2, nature has gotten by without it for 3.8Billion years. Possibly biocatalysts have an equivalent low temperature, low pressure function that will not require iridium, SS or metal, possibly a biobased plastic.
  • #5
Stainless steels and many other corrosion- and chemoerosion-resistant alloys are resistant in relatively mild ambient conditions. It is sufficient for most applications, especially in wet and oxidative conditions.

sCO2 is an nonpolar solvent , used mainly for dissolving nonpolar substances and eventually performing physicochemical processes with them. It is perfect in cleaning of various things. In contact with solids it easily removes any organic layers, usually protecting these solids from water contact and water-mediated corrosion.

Some sCO2 soluble organic compounds are prone to formation of coordination "complex" compounds with metal ions, removing primary formed metal ions from solution. This shifts equilibria between solid metal and fluid solutions in the direction of dissolving metals. This is dissolution, not a classical corrosion, because of the lack of corroded layer on metal.

When you are not interested in ionic equilibria in nonpolar solvents, and need only corrosion- resistant material, try to cover your corrosion-sensitive surface with diamond-like carbon layer, or various glasses, also very thin.
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