Materials Performance

OCT 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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18 OCTOBER 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 10 MATERIAL MATTERS Continued f rom page 17 The stressed faces of uncoated (top) and CVD WC-coated (bottom three) samples of 17-4 PH SS after a sulfide stress cracking test. Photo courtesy of Hardide Coatings. practical, technically, and commercially viable solution to HCP coatings that are capable of increasing component and tool life. These CV D coatings belong to a fam- ily of nanostructured W/WC coatings. They are crystallized atom-by-atom from low-pressure gas media. Crystallization at an atomic level produces a uniform, pore-free coating because the highly mobile reaction products f ill micropores and defects in the coating as it grows. The gas phase of the CV D process enables internal surfaces and complex designs to be coated, which allows this technique to coat "out of line-of-sight" surfaces and geometries where spray coatings cannot be used. Due to the deposition mecha- nism, CV D WC coatings have exception- ally low porosity and do not need to be sealed. The dispersed WC nanoparticles give the material enhanced hardness, which can be controlled and tailored to give a ty pical range of between 800 and 1,600 Vickers hardness (H V) for different coating ty pes. The CV D coating can be produced in thicknesses ranging from 5 to 100 µm, and can match the thickness of HCP, thus simplif ying the replacement process. By varying the ratio between the thickness and properties of each individ- ual layer, the overall coating characteris- tics can be adjusted to meet specif ic application requirements. The hardest CV D WC coating (3,000 to 3,500 H V), Ty pe H, consists of pure WC, which is extremely hard and has limited thickness (5 to 12 µm) and satisfactory toughness. The multilayer CV D WC coating, Ty pe M, includes layers of various hardness and exhibits good toughness. The most widely used ty pe of CV D WC coating is Ty pe T, which consists of WC nanoparticles dis- persed in a tungsten matrix. This struc- ture gives it a unique combination of properties: high hardness (from 1,100 to 1,600 H V) combined with excellent toughness for impact and crack resis- tance. The CV D WC coating developed to replace HCP, Ty pe A, matches its hard- ness and thickness. Both W and WC are highly chemical resistant and protect against mineral acids that include hydrochloric acid (HCl) and sulfuric acid (H 2 SO 4 ). Additionally, the CV D process—unlike sprayed H VOF coatings—does not use cobalt, which can be affected by acid. As a result, the non- porous CV D WC coatings are resistant to many aggressive chemicals and can be used as an effective anticorrosion barrier for critical parts. Several tests were run to confirm the CVD WC coatings' performance. In accor- dance with ASTM B117, 1 mild steel plates coated with HCP, HVOF, and CVD WC Type T coatings were subjected to a neutral salt spray. The HCP samples became very cor- roded after 288 h of exposure and were removed from the test. After 480 h, the HVOF-coated samples showed heav y rust stains and the coating blistered due to inten- sive corrosion of the steel plate beneath. The CVD-coated samples showed light staining. In accordance with NACE TM0177 2 and ASTM G39, 3 the CV D WC Type T coating was tested for sulfide stress cracking by Bodycote Materials Testing for resistance to aggressive media. One uncoated and three coated samples of 17-4 PH (UNS S17400) stainless steel (SS) and also coated 316 SS (UNS S31600) and Inconel 625 (UNS N06625) samples were tested in deformed conditions (with coating elongation up to 3,000 microstrain) in a 5% sodium chloride (NaCl)/5% acetic acid (CH 3 COOH) solution saturated with H 2 S. During the test, the uncoated sample cracked completely across its 20-mm width and experienced extensive microcracking and pitting, while the CV D WC-coated sub- strates showed no microcracking, macro- cracking, or degradation. This demon- strated that the coating can help extend the life of valves or pumps controlling sour oil and gas in both upstream and downstream applications. To test galling resistance of the CV D WC coating, a Phoenix TE77 high- frequency reciprocating test rig was used. This test uses the reciprocating dry sliding movement of a cylinder on a f lat plate with loads gradually increasing from 10 to 800 N (equivalent to contact pressure of 810.2 MPa). It monitors the coeff icient of friction (CoF), where a value above 1.0 indicates severe galling. In comparison with a base- line control test using a SS pin, which was stopped due to sample seizure after reach- ing a 1.0 CoF value with a 65-N load, a CV D WC-coated pin was tested against a coated plate, and the dry CoF remained low, stabi- lizing at around 0.2. No galling was observed, even under the maximum load the test rig could produce.

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