Valve Magazine - Spring 2011

coatings have proven to provide satisfactory results.

Thermal spray coating is a series of processes that apply a consumable in the form of a spray of finely divided molten or semi-molten droplets to produce a coating. The characteristics that distinguish thermal spray processes from weld overlay are as follows:

; A mechanical bond occurs between the coating and the substrate.

; Spray deposits can be applied in thinner layers than welded coatings.

; No degradation occurs on the substrate with thermal spray coatings.

a metallurgical bond is required, other coating processes need to be utilized for the application of wear-resistant type coatings. These processes can include plasma transferred arc and laser.

CONCLUSION
With environments becoming more
severe, special attention is needed in
choosing materials for subsea applica-
tions. This ranges from selecting the
best body material to meet the required
life of the project to the small details
required of cap screws. Selecting the
main valve materials often is not an
issue because of the extensive history
available on subsea equipment. Special
attention, however, is required on
details. This includes appropriate con-
trol in areas of galvanic, pitting and
crevice corrosion, as well as hydrogen
embrittlement of materials from the CP
system. These details must be taken into
account to ensure correct selections. VM

MANUEL MALIGAS is an independent consultant in materials and corrosion for oil and gas, and a fellow of ASME International. He retired after 23 years with FMC technology where he was senior material specialist. Reach him at metengr@sbcglobal.net.

The most common thermal spray process used for coatings on wellhead equipment is that of the high-velocity oxygen fuel (HVOF) process. With HVOF, coatings are projected on the substrate in the semi-molted condition at high velocities—resulting in high hardness, strong bond strength and high durability. Common coatings applied using HVOF include 88WC-12Co, 83WC-17Co. These types of coatings are used for wear, although they are also required to be resistant to corrosive conditions to which they’ll be subjected. HVOF coatings can be applied to any substrate material.

HVOF coatings, although they have strong bond strength, do not have a met-allurgical-type bond with the substrate. Thermal spray coatings can have porosity, and thus the substrate should be corrosion resistant as well. In cases where

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REFERENCES

1. M. Maligas, E. Hibner, “Highly Corrosion Resistant Weld Overlay for Oil Patch Applications,” Corrosion/94, Paper 62 (Houston, TX: NACE International).

2. M. Maligas, “Use of Corrosion-Resistant Weld Overlays for High Pressure Applications,” OTC 7521, Offshore Technology Conference 1994.

3. M. Maligas, J. Vicic, S. Olsen, and P. Nice, ”Material Selection for Wellhead Equipment Exposed to Chlorinated and Natural Seawater,” Corrosion/96, Paper 80 (Houston, TX: NACE International).

4. G. Byrne, R. Francis, G. Warburton, M. Maligas, “The Selection of Superduplex Stainless Steel for Oilfield Applications,” Corrosion 2004, paper 04123.

5. M. Maligas, P. Woolin, “Testing of Superduplex Stainless Steel for Sour Service,” Corrosion 2003, paper 03132.

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