case study
Figure 1 demonstrates the stainless steel options for the various environments versus the corrosion resistance (using an empirical pitting resistance formula, derived from our laboratory pitting test results) and cost. In this specific case study, instead of using the conventional wisdom detailed above, an alternative approach was taken. Note: If the material selection criteria are low corrosion resistance and structural integrity, then 3CR12 is the solution. This is a proven low-cost stainless steel solution with a fifty-year track record in all atmospheric environments. Figure 2 shows the results of a 20 year exposure programme that was conducted in various corrosive atmospheres around South Africa. Although the actual corrosion rate of 3CR12 varied from 0.001 μm/annum in a rural environment to 1.6 μm/annum in a severe marine environment, the relative corrosion resistance of the various metals was quite consistent. Based on this exposure testing, 3CR12 would be expected to last about 150 times longer than mild steel. From a structural integrity (pitting) point of view, the maximum pit depth observed in 3CR12 was in a coupon in a severe marine environment and even in this case, the pit depth was only 0.25mm after 10 years.
With atmospheric corrosion applications, material selection based on these considerations may lead to the following generalised conventional wisdom, summarised in Table 1: according to SANS10400-L
Table: Minimum grade for stainless stee by SANS 10400-L
Corrosivity Category
CSIR Classification/ ISO 9223
STAINLESS STEEL
C1
Desert
3CR12L and 430
C2
Rural/semi-desert
C3
Inland industrial
441/304/436/2001
C4
Medium severe marine
444
C5
Marine industrial
316 and 2304
CX
Severe marine
2205
This material selection philosophy traditionally results in structures and fabrications built with good corrosion resistance and stain resistance. But it can be a costly solution.
Figure 2: CSIR research data on the Relative Atmospheric Corrosion Resistance
3CR12 is a 12% chromium-containing ferritic stainless steel, but unlike normal ferritic stainless steels, it is tough, even at temperatures as low as -30°C - and this toughness is retained even if it is welded. Conventional ferritic stainless steels exhibit low toughness (DBTT) and limited weldability and can therefore only be used in cold rolled applications with a maximum thickness of 3mm. 3CR12 as well as the austenitic (304L and 316L) and duplex stainless steels (2001, 2304 and 2205) are all tough, even when welded and can therefore be used in all thickness ranges.
Figure 1: Corrosion Resistance vs. Price
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Issue 3 – 2023
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