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Tubing: Corrosion Resistance Offshore The environmental conditions in which engineering products have to operate are by their nature fairly arduous.
None poses greater or wider problems than sea water and the associated saline atmosphere.
The life expectancy of equipment is often drastically reduced. In areas where corrosion has already taken place, predicting failure as a basis for preventative maintenance is difficult, if not impossible.
At its most aggressive, in the presence of oxygen, sea water attacks most ‘standard’ materials in a variety of differing but equally costly ways: –
Carbon Steels Prove almost impossible to protect and corrode away almost immediately.
Common Copper Alloys Often suffer a shortened life, either as a result of dezincification or stress corrosion.
Stainless Steels – 304, 316 Stainless steels are highly prone to chloride pitting, a particularly damaging form of crevice corrosion that leads to the creation of small holes in the material.
Super Duplex and Austenitic Stainless Steels – 904L, Monel, 6Mo-Alloy 400, and 2507 Although these materials are far more corrosion resistant than stainless steels, they are not corrosion proof and do suffer pitting and crevice corrosion after time.
Tungum alloy tubing C69100 Tungum tubing has been developed to counter the problems of sea water corrosion and has been used extensively for over 40 years. Whether totally immersed, or in the highly active ‘splash’ zone, Tungum alloy tubing gives exceptional resistance to the effects of a marine environment. Unaffected by either pitting or crevice corrosion, Tungum alloy tubing is protected by its oxide coating. This oxide coating quickly repairs itself if damaged.
Tungum tubing remains unscathed despite more than 10 years marine exposure on a semi-submersible support vessel. The stainless steel section from a southern North Sea platform, shows both crevice corrosion and chloride pitting after barely 5 years in the same environment.
Protective Oxide Coating The special corrosion resisting characteristics of Tungum tubing, carefully developed for use in the hydraulics systems of marine aircraf,t remains just as valid in today’s polluted sea waters.
The development of the oxide coating is illustrated by the graph.This shows time plotted against a minute weight loss during its formation.After 1000 hours the weight has virtually stabilised indicating that the protectivecoating is already almost complete.A fact confirmed in the laboratory by the most rigorous tests and backed by experience of countless practical and demanding applications. Many of these more than 50 years standing.
As a result of the excellent corrosion resistance characteristics of Tungum and integrity management feedback received, a corrosion allowance is not generally required for Tungum tubing when used in suitable applications
Tungum corrosion testing results can be found in various articles including: NACE Testing NACE Paper No. 10305 entitled ‘316 Stainless Steel Instrument Tubing in Marine Applications – Localized Corrosion Problems and Solutions.’ This paper compares various metallic materials in addition to Tungum Alloy (UNS C69100) including 316L, 317LMN (UNS S31726), Alloy 825 (UNS N08825), 6Mo (UNS S31254), Alloy 625 (UNS N06625) and underpins the historical evidence of successful performance in these demanding safety critical applications. Materials were laboratory tested in a cyclic salt fog chamber to ASTMD5894 at temperatures ranging from 25 to 45 degrees C and also fieldtested by exposure to a marine environment for a one year duration onboard two offshore platforms; one located in the Gulf of Mexico and theother in Trinidad. In the laboratory tests Tungum Alloy (UNS C69100) was the material that resulted in the best localized corrosion resistance judging by two of the three visual inspection indicators
Conclusions were that Tungum Alloy (UNS C69100) is a material that can be safely used in a marine atmosphere keeping in mind its pressure and internal corrosion limitations.The results of the test were published in NACE Paper No. 10305 and presented at the NACE CORROSION 2010 Conference & Expo. A copy of the paper can be obtained direct from: NACE International website
Various corrosion related tests and calculations including PREN, G48 and CPT can be applied to stainless steel and related nickel and chromium based alloys, but none of these tests are appropriate for Tungum as it is a copper based alloy, as explained below.
PREN (Pitting Resistance Equivalent Number). NACE and NORSOK typically require a PREN of 40 minimum for oil and gas applications. Normally calculated using the formula: PREN = wt%Cr 3.3wt%Mo + 16wt%N. This formula, however, cannot be applied to Tungum because it is a unique copper based alloy and does not contain any Molybdenum or Chromium.
ASTM G48 Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys. By definition this standard does not apply to copper based alloys.
CPT (Critical Pitting Temperature) for Stainless Steels and Related Alloys. This can be determined either by ASTM G48 method C or E. Alternatively, using electrochemical test methods to ASTM G150 Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels. Again, by definition these standards do not apply to copper based alloys.
Excellent resistance – minimal attacks take place – for the following substances:
Good resistance under the conditions of test were achieved for the following substances:
These ratings are the result of laboratory tests conducted in-house under the controlled conditions noted. They are published for guidance only. Where any doubt exists, samples of Tungum are freely available for field trials to replicate the precise operating conditions.
Tungum tubing has a high level of general corrosion resistance, allowing it to be specified for use in systems containing, or operating in the presence of a variety of substances and solutions.
