Thermodiffusion Galvanizing of Tubing Couplings

Our factory embrace work aimed at increasing the service life of tubing. In this field, certain positive results have been achieved.

One such method for improving the corrosion and wear resistance of products for the oil and gas industry is the thermodiffusion galvanizing (TDG) process.

The process involves heating of metal products in a mixture with zinc-containing powder. The diffusion character of the process affects the structure of the coating that is a metal layer, in which the content of zinc decreases as the base metal is approached, while the content of iron accordingly increases. Only the outer layer of the coating consists of pure zinc. The coating being formed on the metal surface has high adhesion for steel, features high weather-resistance in all climatic regions, as well as resistance to water and most organic liquids exposure. In this case, the hydrogen embrittlement is eliminated completely.

The coating meets the requirements of GOST 28426-90 and GOST 51163-98 and of foreign standards BS 4921: 1998, ASTM B663, ASTM B695. As to its performance, including the wear-resistance, the thermodiffusion coating outperforms electrodeposited coating that was confirmed by the tests No. 623.01 made by the OAO VNIITNeft in February, 2002.
In the investigation, type 73 couplings per GOST 633-80 with triangular thread with thermodiffusion zinc coating were tested for screwing and unscrewing using the thread sealing grease PSM VNIITNeft per TU 0254-142-0147016-01.

The test for screwing - unscrewing was performed on the test stand SINKT-450; when screwing, the torque was from 900 to 1,500 Nm, and the threaded connection was additionally loaded by an axial compression force of 1,000 N simulating the load induced by the weight of one tube of a tubing string.

In the test, the threads of the coupling and the nipple were visually inspected for scoring, galled thread, and abrasion of the coating on the thread of the couplings.

The coupling-to-nipple threaded connections were tested for leaks at a hydraulic internal pressure of 59.4 MPa before and after the tests for screwing - unscrewing.

The threaded connections were under the test pressure for 15 minutes. There was no visible leakage in the wall and by the thread of the nipples and couplings.

The type 73 couplings per GOST 633-80 with triangular threads with the thermodiffusion zinc coating withstood 54 screw - unscrew cycles, with the tightness and the geometry of the thread being retained.

The test of the coupling coated by the thermodiffusion galvanizing method was terminated since these couplings obviously offered an advantage over other types of the couplings being tested.

At present, the thermodiffusion galvanizing process for coating the couplings manufactured by FGUP

Nevyansk Mechanical Works has been mastered. We are ready to supply the tubing couplings of various diameters and strength groups conforming to GOST 633-80.

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