Corrosion Resistance of Tungsten Carbide Grades

Corrosion Resistance of Tungsten Carbide Grades are by far the most widely used binder metal or "cement” in cemented tungsten carbides because it most effectively wets tungsten carbide grains during liquid phase sintering. For this reason Corrosion Resistance of Tungsten Carbide Grades are believed to be superior to other binder metals in terms of eliminating residual porosity and achieving high strength and toughness values in sintered products. The failure of cemented carbides in corrosive environments, however, is generally due to the chemical reaction of cobalt with corrosive agents. The corrosion process involves the dissolution of the cobalt binder at exposed surfaces leaving a loosely knit skeleton of tungsten carbide grains having little structural integrity. This mechanism is often referred to as cobalt "leaching” and is typically accompanied by a flaking off of unsupported carbide grains in the affected surface areas.
Although WC + Co grades have fairly good resistance to attack by acetone, ethanol, gasoline and other organic solvents as well as by ammonia, most bases, weak acids, and tap water. Exposure to formic, hydrochloric, hydrofluoric, nitric, phosphoric, sulfuric, and other strong acids, however, can result in a relatively rapid deterioration of the binder phase. Corrosion rates are affected also by temperature, the concentration and electrical conductivity of the corrosive agent, and by other environmental factors. Alloying cobalt with chromium, molybdenum, and/or nickel can significantly improve the resistance of the binder to corrosion, but the complete substitution of nickel for cobalt has proven to be the most effective means of extending the life of cemented carbides in highly corrosive environments. WC + Ni grades and other special formulations designed to resist corrosion without compromising strength or other attributes are therefore ideal for applications such as seal rings, fluid control components, choke valves, nozzles, and bearings.

Specifications

PH	WC + Co Binder	WC + Ni Binder
12	Very Good	Very Good
11
10
9	Good
8
7	Fair
6	Poor
5	Very Poor to No Resistance to Corrosion	Good
4
3		Fair
2
1		Poor
0

Features

Solution Type and Temp.	Liquid Agent	Concentration (%)	FC3 (6% Co)	FC10 (9% Co)	FC3N (6% Ni)	FC8N (Ni-Cr-Mo)
Acids at 50°C.	Acetic acid	10	G	G	G	VG
		50	G	G	VG	VG
		100	G	G	VG	VG
	Boric acid	10	G	G	VG	VG
	Chromic acid	10	G	G	G	G
	Citric acid	10	G	G	VG	VG
	Formic acid	10	F	F	G	G
	Hydrofluoric acid	20	G	G	G	VG
	Hydrochloric acid	5	F	F	G	F
		35	G	G	VG	G
	Nitric acid	5	P	VP	G	G
		50	VG	VG	VG	VG
	Phosphoric acid	10	P	P	G	G
		50	P	VP	F	F
	Sulfuric acid	10	P	VP	G	G
		50	G	F	VG	G
		90	G	G	VG	G
Alkalis at 50°C.	Potassium hydroxide	10	G	G	VG	VG
	Sodium hydroxide	10	G	G	G	VG
Salt Solutions at 50°C.	Ammonium chloride	10	G	G	G	VG
	Ammonium nitrate	10	F	F	F	VG
	Ammonium sulfate	10	G	G	G	VG
	Hydrogen peroxide	10	VP	VP	VP	F
	Potassium hydroxide	10	G	G	VG	VG
	Potassium silicate	10	VG	VG	VG	VG
	Sodium chloride	10	VG	G	VG	VG
	Sodium hydroxide	10	G	G	G	VG
Water at 50°C.	Sea water	100	G	G	VG	VG
	Pure water	100	VG	VG	VG	VG

Corrosion Resistance of Tungsten Carbide Grades

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