Choosing a wedge wire screen material, from SS 304 to Hastelloy

Corrosion at welds and crevices

A wedge wire screen has thousands of weld points and a long, narrow slot, and each weld and wire to rod contact is a potential crevice. If the grade does not suit the water, pitting starts at these points, widening slots and weakening welds. The first sign is often media or sand in the outlet, well before the screen looks corroded.

The material must therefore suit the chloride level, temperature, pH and chemistry of the process, as well as the loads and abrasion the screen will see in service.

Wedge wire screen material grades

SIMO Screens makes wedge wire screens in six materials. The table gives the nominal composition and the typical pitting resistance equivalent number (PREN) of each.

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PREN is not the usual basis for ranking nickel alloys, and it is shown for C-276 only to place it against the stainless grades. Monel 400 contains no chromium, so PREN does not apply. Hastelloy C-276 is the SIMO standard Hastelloy grade; other Hastelloy grades are supplied on request, although sourcing them in India can take longer.

The PREN formula

PREN = %Cr + 3.3 x %Mo + 16 x %N

The actual chemistry from the Mill Test Certificate should be used rather than the nominal figure, because two heats of the same grade can differ by a few points.

Worked example

An MTC for Duplex 2205 wire shows 22.5% Cr, 3.2% Mo and 0.17% N.

1. Chromium: 22.5
2. Molybdenum: 3.3 x 3.2 = 10.56
3. Nitrogen: 16 x 0.17 = 2.72
4. PREN = 22.5 + 10.56 + 2.72 = 35.8

For comparison, 316L at 17% Cr, 2.1% Mo and 0.05% N gives 17 + 6.93 + 0.8 = 24.7, so the duplex has about 11 points more pitting resistance.

Chloride guidance as general practice

Widely used guidance for stainless steel in water ranks grades by chloride content, assuming ambient temperature, near-neutral pH, and flowing, aerated water.

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These values are starting points. Several conditions make a process more aggressive than its chloride figure suggests:

• Higher temperature. Pitting and crevice resistance fall as temperature rises.
• Low pH. Acid water attacks the passive film more readily.
• Stagnant water and deposits. Sludge, scale and biofilm form crevices and starve the surface of oxygen.
• Residual chlorine and oxidants. Chlorination raises the corrosion potential and narrows the margin.
• Concentration by evaporation. Wet and dry cycles can concentrate chlorides far above the bulk level.

Where any of these apply, the next grade up in the table is normally selected, or the process conditions are referred to SIMO engineering for review.

Temperature guidance as general practice

Temperature affects the choice in two ways: it accelerates chloride attack, and it sets outright limits on some grades.

• Chloride stress corrosion cracking. Austenitic grades such as 304 and 316L are generally considered at risk above about 60 °C in chloride-bearing water under tensile stress. Duplex grades have much better resistance, and super duplex better still.
• Upper limit for duplex. Duplex and super duplex are normally limited to about 300 °C in service, because longer exposure above this embrittles them.
• High-temperature service. For hot process applications such as reactor internals, the licensor or vessel specification usually names the grade, and that specification should accompany the enquiry.

SS 304 vs 316L and other grade steps

For most buyers the first decision is SS 304 vs 316L, and the chloride level usually settles it. The table sets out each step up the range.

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Typical starting grades by application

The table gives common starting grades as general practice. The water or process analysis decides the final choice.

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Strength and abrasion

Corrosion is not the only reason to change grade, because duplex grades are much stronger than austenitic grades in the annealed condition.

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These are plate values, shown for comparison between grades; wire and rod properties depend on their own product standards and condition. Higher strength allows a duplex screen to carry more load on the same profile, or to keep a larger margin against collapse and impact. In abrasive slurries, wear life depends more on wire size and velocity than on grade alone.

Welding, low-carbon grades and surface finish

Wedge wire is a welded product. Every wire to rod intersection is resistance welded, and there are further welds to end rings, flanges and frames.

Low-carbon grades such as SS 304L and SS 316L reduce the risk of sensitisation at welds; SIMO supplies SS 304L where a welded fabrication or the purchaser's specification calls for it, which matters most where the screen sees acids or warm chloride water. Duplex grades need controlled welding to keep the correct phase balance. Heat tint at the welds is removed by pickling and passivation where the service calls for it; SIMO arranges pickling, passivation to ASTM A380 or A967 and electropolishing through approved agencies on request.

Recording the material on an enquiry

The enquiry should give the grade for each part, because the profile wire, support rods, end fittings and frame can differ, and the wire and rods matter most.

It should also state any test or document requirements, such as the MTC standard, positive material identification or a sour service specification. The Mill Test Certificate is supplied as standard, and PMI is arranged through an approved agency on request. A water analysis or process chemistry allows SIMO engineering to check the grade against the operating conditions.

Related products, tools and engineering support

• Products: intake screens for seawater and river water, water well screens for saline aquifers, and support grids for chemical and refinery vessels.
• Tool: the material selector narrows the grade from chloride, temperature and pH.
• Engineering: for mixed chemistry or high temperature, see custom engineering.
• Further reading: profile wire and support rod selection.