The total annual cost of steel corrosion is $2,500 billion across the globe: 3% of GDP
Steel corrosion review by Geoff Small, Victrex Market Technology Manager
Sulfate-reducing bacteria (SRB) are among the oldest forms of life, having contributed to the sulfur cycle which started around 3.5 billion years ago. There are over 220 species of SRBs and they occur naturally in most anaerobic environments, contribuing to the degradation of organic matter.
Although many bacteria also reduce sulphates in small quantities, sulphur reducing bacteria reduce sulphates in significantly larger amounts. Toxic hydrogen sulfide is a waste product of SRBs, with its rotten egg odor a marker for the presence of sulfate-reducing bacteria in nature, such as the sulfurous odors of salt marshes and mud flats. SRB occur naturally in sour crude oil and seawater and can anaerobically oxidise methane to form sulphide ions, deriving their energy from oxidizing organic compounds or molecular hydrogen.
Apart from localized corrosion risk, protecting any steel structures from SRBs costs the oil and gas industry heavily in chemical treatments such as corrosion inhibitors, biocides and scale inhibitors for carbon steel and pipelines.
“The total annual cost of corrosion is estimated to be $2,500 billion across the globe, representing over 3% of global GDP. Within oil and gas 57% is related to a combination of pipe costs and direct capital expenditure related to directly combatting the effects of corrosion.”
A big challenge in oil and gas is that the hydrogen sulfide SRBs produce reacts with any metal ions in seawater to produce metal sulphides which are insoluble and often black or brown, such as ferrous sulfide (FeS).
Sulfate-reducing bacteria create problems when any metal structures are exposed to sulfate-containing water. The interaction of SRBs at the interface between the metal and creates a layer of molecular hydrogen on the metal surface. Sulfate-reducing bacteria then oxidize the hydrogen and create hydrogen sulfide, which is a significant corrosion problem for any subsea pipe or structure.
When dissolved, hydrogen sulphide forms a weak acid which is corrosive and with steel forms a scale layer of iron sulphide.
Depending upon the operating temperature this scale may be passive or can accelerate galvanic corrosion by acting as an anode.
The additional challenge of the presence of CO2 exacerbates what is already a subsea corrosion risk.
“Both general and localized pitting corrosion rates of carbon steel were found to nearly double in solutions containing 10v% CO2 and 10 v% SRB compared to solutions containing either CO2 or SRB alone”. NACE report
SRB are known for causing anaerobic corrosion of buried pipelines where iron sulphide is formed and accumulated on the surface of steel pipelines, which then accelerates the further dissolution of iron.
SRB bacteria accumulation can lead to pitting of steel and the increased corrosiveness of water, raising the possibility of hydrogen blistering or sulphide stress cracking.
In applications where Victrex PEEK polymer is used in Thermoplastic Composite Pipe (TCP) such as Magma’s m-pipe®, it has a smooth surface which ensures a high resistance to the build-up of any bacteria or other organic matter. As PEEK polymer is highly chemically resistant, it is extremely difficult for SRB to be able to gain a foothold on the surface of the PEEK bore in a pipeline. But PEEK’s benefits don’t stop there. Where SRB bacteria are present then PEEK polymer also exhibits an excellent resistance to high concentrations of H2S and, unlike steel pipe, PEEK s not affected by corrosion, scale formation or hydrogen induced embrittlement.
Victrex has generated a significant data set around PEEK resistance to hydrogen sulphide following International Standards such as NORSOK M710 and ISO 23936. In addition Victrex has exposed PEEK polymer to ageing at various temperatures in a three phase environment where the gas phase is 100% H2S. At temperatures below 175°C there is no effect of this gas on the polymer and only at higher temperatures (>200°C) does a gradual decrease in performance (through chain scission) begin to occur.
Left is a graph plot of tensile strength of Victrex PEEK polymer versus ageing time in the pure H2S phase of a three phase aromatic NORSOK M710 type environment at 125°C and 175°C. This data demonstrates the exceptional sour gas resistance of this polymer.
The smooth surface of products like Magma m-pipe® which utilise Victrex PEEK provide inherent chemical resistance, especially to sour gases, which means that there is now a viable alternative to carbon steel pipe for subsea use that eliminates the expensive costs of corrosion inhibitors, biocides and scale inhibitors.