Influence of Steel Composition on Strain Induced Corrosion Cracking and Other Types of Corrosion

Purpose - To make clear that carbon steels can differ very much in corrosion resistance under practical conditions because of minor differences in the chemical composition of the steels.

Design/methodology/approach - In the electricity generating industry, ‘mild' (i.e. ‘plain carbon') and low-alloyed steels are used in huge quantities, for example in the construction of boilers, steam generators, heat recovery boilers and waste incineration boilers. The resistance to SICC was determined by measuring the “repassivation” behaviour of the steels at freshly ground surfaces with an electrochemical technique. The corrosion current measured with time was used to calculate the cracking rates of a CT specimen.

Findings - A correlation was found between chemical composition, corrosion resistance to SICC and experiences under practical conditions. The results of early published papers on boiler corrosion (testing in FeCl 2 solutions), erosion corrosion (testing in wet steam at 20 bar), Nitrate Stress Corrosion Cracking (testing in NH 4 NO 3 solutions ) and Strain Induced Corrosion Cracking (SICC), together with those originating from in-service failures, were compiled into a reference database. This paper is a compilation and review of that work.

Originality/value - The database and formulae presented make clear there is often a direct correlation between chemical composition of ordinary “C-steel” and these specific types of corrosion failures. The paper is of importance to designers, failure analysts and researchers.

From the results of the research done on C-steel and low-alloyed steels over a long period of time it is clear that the chemical composition of the steels can be a determining factor in the following types of failures:

Elements that where found to be beneficial in preventing these types of failures are included in Table 6.

Type of failures Boiler corrosion caused by chloride ingress under heat flux conditions	Beneficial elements in C-steels	Non beneficial elements	Failure mechanism
	Mn, P, Cr and Mo		Formation of compressive stresses in the magnetite layer
Erosion corrosion in wet steam lines	Cu, Cr and Mo	C	Porosity of the magnetite layer
Nitrate Intergranular Stress Corrosion Cracking	Mo, Cr and Mn	Si, Cu and C	Intergranular corrosion
Strain Induced Corrosion Cracking (in deaerators)	Mo	C	Repassivation capacity on the crack tip

For a more reliable operation of equipment, designers therefore are advised strongly to specify carbon steel and low-alloyed steels with tighter compositional specifications than previously was usual.

Table 1 - Chemical compositions of the steels tested in the electrochemical repassivation measurements

No	Steel	Cr	Cu	Mo	C	Si	S
A	St 35.8	0.08	0.323	0.033	0.118	0.21	0.028
B	13CrMo4.4	1	0.1	0.42	0.13	0.3	0.03
C	15Mo3	0.02	0.04	0.28	0.16	0.009	0.015
D	St 35.8	0.08	0.14	0.26	0.17	0.22	0.02
E	St 35.8	0.01	0.02	0.001	0.18	0.32	0.024
F	14Mn4	0.1	0.1	0.03	0.1	0.4	0.02
G	15Mo3	0.11	0.07	0.29	0.14	0.23	0.01
H	10CrMo9.10	2.3	0.1	1	0.09	0.2	0.01

Figure 5 - Calculated crack growth of a CT specimen based on the electrochemical measurements in a sodium sulphate hydroxide environment for the 8 tested steels. For one condition (P=1000 N) the cracking rates of the 8 steels in Table 1 have been calculated. Figure 5 shows the cracking rates for increasing crack lengths .