Intergranular Corrosion Properties of Alloy C276 and Alloy 22/C22

ASTM G28A and G28B Corrosion Tests

Development over the past 70 years of the “C” family of alloys has been focussed on improvements in alloy metallurgy, in melting technology and in thermo-mechanical processing. ASTM G28A and G28B tests where developed later as qualitative corrosion tests to verify that the metallurgy, melting process and thermo-mechanical processing had been completed correctly to ensure that the material is in the fully solution annealed condition and is not subject to intergranular corrosion resulting from the presence of intermetallic / carbide precipitates at the grain boundaries. There is no pass or fail criteria for the tests. The results are agreed between buyers and suppliers.

ASTM G28A

Ferric sulphate and sulphuric acid boiling test
50% H₂SO₄ + 42g/litre Fe₂ (SO₄)₃

ASTM G28B

Mixed acid (Sulphuric acid, hydrochloric acid, ferric chloride & cupric chloride) – oxidizing salt boiling test.
23% H₂SO₄ + 1% HCl + 1% FeCl₃ = 1% CuCl₂

Published industry accepted corrosion rates for C276 and C22/Alloy 22

Producers	Alloy C276 G28A (mm per annum)	Alloy C276 G28B (mm per annum)	Alloy 22/ C22 G28A (mm per annum)	Alloy 22/ C22 G28B (mm per annum)
Haynes International	6.10	1.40	0.61	0.178
Special Metals	7.00	1.50	1.50	0.50
ThyssenKrupp VDM	9.00	1.50	0.90	0.17

Results based on taking the average of a minimum of 10 mill test certs/heats

Producers	Alloy C276 G28A (mm per annum)	Alloy C276 G28B (mm per annum)	Alloy 22/ C22 G28A (mm per annum)	Alloy 22/ C22 G28B (mm per annum)
Haynes International	6.172	Not reported	0.807	0.145
ATI ( Allegheny)	5.985	1.330	0.822	0.151
Special Metals Huntington , U.S.A	4.315	0.736	–	–
Special Metals Wiggin, UK	6.835	0.858	1.135	0.143
BGH – Germany	4.050	0.780
ThyssenKrupp VDM	–	–	0.809	0.172
Bohler	–	–	0.590	0.094
Special Metals Huntington. U.S.A.	–	–	0.821	0.119