The questions in this issue feature, the possible corrosion impact of oxygen, and use of different zinc-based coatings.
Question:
In an oil and gas development I am working on in the Far East, a concern has been raised about possible corrosion impact due to excess oxygen. The current plan for gas lifting wells is to use N2 with 96% purity, with the remaining 4% oxygen. With operating pressures of 2300 – 2800 psig (at 1 mmscfd), this means that there will be a high level of O2 seen in the produced fluids. The production casing is carbon steel (CS) and the tubing material, 13 Cr steel is used in two wells, and carbon steel used in one well. The N2 gas will be delivered to well head platform via 850 m of flexible flowline (plastic material designed for high pressure) and then injected down the annulus. The production from the wells flows to a FPSO for further processing.
We are looking to reduce oxygen in gas phase and currently thinking of using an oxygen scavenger. Appreciate your expertise to advise on how these issues can be overcome. CM
Answer:
With this application there are two different issues to discuss.
• 13CR corrosion in the tubing
With 4% oxygen in the injected gas at these high pressures it would be expected that the dissolved oxygen in the production stream would be very high, and that the aqueous phase to be fully saturated with oxygen. In theory, some oxygen is helpful for CRAs (Corrosion Resistant Alloys) as they can form a protective film with oxygen. This complex film formed contains the structure, -Cr-O-Cr – O-Cr-, with chromium surrounded also by water ligands, and this makes the excellent protective film on the metal surface. However, it has been known (especially in the presence of high chlorides with 13Cr) that high oxygen may lead to deep pitting. This is because chloride gets inserted in this chromium oxide complex and weakens it, which can lead to the rupture of the protective film. The corrosive fluids with oxygen can now contact the metal surface and lead to serious pitting corrosion. As noted, a small amount of oxygen is a good thing for CRAs, but higher levels may lead to failures.
• Carbon steel corrosion
Carbon steel tubing in the annulus and one of the wells may undergo heavy corrosion in the presence of oxygen. Oxygen generally is more corrosive than CO2 or H2S. The by-products (magnetite or hematite) from oxygen corrosion are not protective, so will not protect the steel under any circumstances. This is a situation (very high oxygen) where oxygen scavengers are not cost effective and are also very difficult to apply to a gas stream. Although not an easy option, inhibition can be used in some instances for oxygen corrosion, but It is important that the product be selected in the correct manner as not all inhibitors are effective against oxygen corrosion.
In this situation, the best overall solution would be to reconsider the gas used for the gas lift system and use a source that does not contain oxygen. This would solve the issue in both annulus and tubing, although if other corrosive species are present CO2 /H2S this would need to be assessed as well. Even with dry gas if there is fluid in the annulus of the well, the gas will get re-saturated with water during injection, so would not be a solution. If this is not an option, inhibition could be considered, although it is very important to use a good oxygen corrosion inhibitor and inject so it that it will inhibit both the annulus and production fluids. The product (s) need to selected correctly to account for the oxygen present, and the application needs to ensure that distribution in the gas and water phase in the product fluids occurs.
George Winning, Clariant Oil Services UK Ltd
Question:
“Zinc-based coatings can provide long term corrosion protection for steel substrates. What is the difference in zinc thickness needed between galvanising, thermal metal spray, and zinc-rich coatings, to give maximum life to first maintenance in different exposure conditions, and what are the cost implications?” K McKee
Answer:
A good question but the answer is probably more complex than you might expect!
Firstly, the corrosion protection system selected needs to be able to achieve the required corrosion protection in the given service environment. On this basis, zinc rich paints alone would not be widely used for primary corrosion protection, and where they are used, it is probably limited to category C1 (low corrosivity environments) as described in ISO 9223. There is no specific guidance on the coating thickness for a zinc rich paint, although they are typically used for touch-up and repair of small areas of damage to a galvanized coating where a coating thickness of 100 microns is required. The cost of application may be less than alternative systems but then the level of protection provided is limited with more frequent maintenance schedules being required!
Batch hot dip galvanizing (e.g. galvanizing after fabrication) is the most commonly used means of applying a zinc coating for corrosion protection. The process is conducted to BS EN ISO 1461 with the coating thickness requirement varying from 45-85 microns dependent upon steel section thickness, although thicker coatings up to 140 microns may be specified if steelwork is grit blasted before hot dip galvanizing. For most atmospheric applications an 85 micron galvanized coating will typically achieve a coating life of 60 years in many atmospheric environments, although thicker coatings may be specified for more corrosive environments. The cost for hot dip galvanizing is charged on a pounds per tonne basis using the as-galvanized weight of the article. Costs may vary according to the size and weight of an article and the volume of work to be processed.
Thermal spraying tends to be conducted on large solid structures which are literally too big to hot dip galvanize and while a zinc coating can be applied, often a zinc-aluminium or aluminium coating is preferred. The process is covered by BS EN ISO 2063 and typically a minimum coating thickness of 100 microns or more would be specified, although this may be increased significantly for corrosive environments. A final point to note is that due to a thermal sprayed coating not being fully dense, an organic sealant often has to be applied. The cost of thermal spraying is typically greater than that for hot dip galvanizing, in part due to the need for steelwork to be grit blasted, the application of an organic sealant, and
the limited number of coating applicators.
Dr Desmond Makepeace, Technical Executive, Galvanizers Association
