Corrosion Resistance

Tungum has excellent anti-corrosion properties

Tungum Tubing has a significantly longer lifetime than Stainless Steel and Super Duplex tubing, due in part to its excellent anti-corrosion properties. Stainless Steel and Super Duplex materials are prone to Pitting & Crevice Corrosion (see image right).

In salt-laden marine atmospheres, ‘316’ stainless steel for example, is highly susceptible to crevice corrosion and chloride pitting. After just a few years of salt spray exposure, it may still look bright from a distance, but closer inspection reveals telltale signs of imminent failure to hold pressure.

Tungum alloy, however, possesses a natural protection mechanism whereby, on exposure to salt spray, a very thin oxide coating is generated over the exposed surface, no more than two thousandths of an inch thick, when complete. The tube becomes discoloured, it may even have a verdigris coating, but under the oxide layer the tube material is perfect and will remain so for the lifetime of the installation.

The image above is an example of Tungum Tubing after 10 years at sea on a dredger. Only the grime and oxide coating has been removed from the tube exterior.

Corrosion Testing

Tungum corrosion testing results can be found in various articles including: NACE Paper No. 10305 entitled ‘316 Stainless Steel Instrument Tubing in Marine Applications – Localized Corrosion Problems and Solutions.’ This paper compares various metallic materials including 316L, 317LMN (UNS S31726), Alloy 825 (UNS N08825), 6Mo (UNS S31254), Alloy 625 (UNS N06625) and Tungum Alloy (UNS C69100) 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 ASTM D5894 at temperatures ranging from 25 to 45 degrees C and also field tested by exposure to a marine environment for a one year duration on board two offshore platforms; one located in the Gulf of Mexico and the other 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 (pit depth in the body of the tubing and crevice corrosion depth). 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

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.