Avoid costly materials mistakes

Common oversights keep plants from getting the most reliable and suitable vessels.

By Chip Eskridge, Jacobs, Steve Zoller, Enerfab, and Mike James, DuPont

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4. Specify dual-grade stainless steel. There’s much confusion about “L” grade, straight grade, and dual-grade 300-series austenitic stainless steels — in particular, Types 304 and 316. Engineers often specify lone “L” grade materials such as Type 304L or 316L on data sheets. The reason: during welding, such low-carbon stainless steels resist chromium carbide sensitization that can lead to preferential heat-affected zone corrosion in some corrosive processes [3]. However, L grade stainless steels have lower strength than straight (non-L) grade stainless steels and the ASME code penalizes the design 15% to 20% with additional shell thickness and lower flange rating [4, 5]. What’s important to understand here is that a lot of the weldable forms of stainless steels (Types 304/316) produced today in the U.S. come dual certified as Type 304/304L or Type 316/316L. These steels have the higher strength of straight-grade stainless steels and have the superior resistance to sensitization during welding of the L grade stainless. This is because they’re now made in a melt furnace process that substitutes nitrogen for carbon. Nitrogen strengthens the steel (like carbon) but won’t promote sensitization during welding. Fabricators often will purchase dual certified materials but will use the lower strength values of the L grade material in their calculations if you specify L grade material on your data sheet. This results in unnecessarily adding extra wall thickness and possibly crossing into a higher flange rating.

5. Properly use corrosion allowance. This allowance adds extra thickness to account for uniform metal loss over the equipment’s expected service life. The key word here is uniform. Mild carbon steel uniformly corrodes due to the galvanic cell potential of the interlaced ferrite-cementite grain structure, called pearlite (Figure 1). Specifically, there are millions of anodic (ferrite) and cathodic (cementite) sites that in the presence of moisture provide the four necessary elements for corrosion (anode, cathode, metallic bridge, and electrolyte). Alloyed materials in aggressive service will also uniformly corrode because their strong protective oxide layer is breached.

Specifying a corrosion allowance for these situations is appropriate. However, many alloys, such as austenitic stainless steels, duplex stainless steels, nickel alloys and titanium, are more resistant to uniform corrosion and tend to corrode locally — that is, pit or crack. So, it’s less appropriate to specify a corrosion allowance for these materials in relatively benign processes. Furthermore, as the thickness of the stainless steel increases, the more likely it can become sensitized from repeated heat input during multi-pass welding. While a mere 1/8-in. corrosion allowance doesn’t seem like much, it potentially can require a disproportional number of additional weld passes (and cost) depending on the weld procedure used.

Figure 1

Figure 1. Adding a 1/8-in. corrosion allowance in this case required an extra 36 inside and 36 outside passes.

A corrosion allowance isn’t recommended for materials that are susceptible to stress corrosion cracking (Figure 2) in a given process. For example, for protection against chloride-induced stress corrosion cracking, it would be more appropriate to upgrade the material of construction to a duplex, lean duplex or super duplex stainless steel, rather than add a corrosion allowance to austenitic stainless steel. Specifying some duplex alloys actually can provide a cost savings because they have 20% to 35% higher allowable code stresses, resulting in a thinner wall vessel [4].

Figure 2

Figure 2. Many stainless steels and alloys resist uniform corrosion but are susceptible to localized attack.

In summary, when uniform corrosion is expected, specify a corrosion allowance. When localized corrosion is expected, investigate other corrosion protection schemes.

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