Susceptibility of Type 304/304L and 316/316L austenitic stainless steels to chlorides in cooling water
Both 316 and type 304 (L is low carbon for weldability) are susceptible, under certain conditions, to three forms of attack in cooling water containing chlorides: pitting, stress corrosion cracking (SCC), and crevice corrosion.
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Each of these have their own criteria in terms of chloride concentration, temperature, etc. where one or more of these forms of attack may initiate and propagate. A brief summary of each is given.
With specific regard to heat exchanger tubing, typical wall thicknesses are 0.065” (16 BWG) or 0.049” (18 BWG) with tube outer diameters 0.625” to 1.0”. Any scale formation on the tube wall (either inner or outer) exceeding the design fouling factors will further elevate the temperature profile and aggravate the effects of chloride corrosion. Some deposits such as calcium carbonate have inverse (retrograde) solubility, preferentially forming on hot surfaces. Chlorides can concentrate underneath these deposits.
Critical Pitting Temperature (CPT)
The CPT is the lowest temperature where pitting can initiate. In Figure 1,1 it is shown that at a chloride concentration of 300 ppm, type 304 stainless steel has a CPT of 40ÂºC (104ÂºF). This is lower than most cooling water return temperatures in summer. Type 316 (with approximately 2.5% molybdenum) has considerably greater pitting resistance. At 500 ppm chloride concentration, the CPT of type 316 stainless steel is 70ÂºC (158ÂºF). A duplex stainless steel such as alloy 2205 has even greater resistance to pitting.
Stress Corrosion Cracking (SCC)
SCC refers to cracking in metals subjected to a corrosive environment. For 316L stainless steel, Figure 22 shows that the threshold temperature at 500 ppm chlorides is approximately 55ÂºC (131ÂºF).
Stresses may be residual, such as those from cold working or welding. Pits themselves also act as “stress risers” and can initiate SCC. This could result in catastrophic failure of a heat exchanger if large numbers of tubes fail simultaneously.
Interestingly, there is no real difference between type 304 and type 316 stainless steel in terms of resistance to SCC, which can occur even at very low chloride concentrations. The threshold temperature for SCC for these alloys is 50ÂºC (122ÂºF) at chloride concentrations of > 100 ppm.
A crevice is a very small gap such as at a tube-to-tube sheet joint. Crevice corrosion can also occur under gaskets, seals, inside cracks, etc. For type 316 stainless steels, the critical crevice corrosion temperature (CCCT) is even lower than for pitting or SCC under similar test conditions and can occur at temperatures as low as 28ÂºF.3
Corrosion Comparison Between Hastelloy C 276 and 316L
Hastelloy C has a CPT of 110ÂºC (230ÂºF) and a CCCT of 105ÂºC (221ÂºF) which is much higher than those for 316L. There is also a number referred to as Pitting Resistance Equivalent Number (PREN). For type 316 stainless, this is 24 vs. 45 for Hastelloy C 276.4 Higher PREN’s indicate increased corrosion resistance. Hastelloy C 276 is considered suitable for use even in seawater.
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