A stainless steel site scoring worksheet

At a glance

Points and bands are IMOA's (ABC 00/03, 2003); layout and wording are ours. Score each component separately where finish, washing or exposure differ. Totals below 0 are not covered by the brochure: we read them as the 304 band.

How to use the worksheet

Our sheet on 304 or 316 stainless steel gives a first filter and shows the bands. This sheet is the full worksheet. It follows IMOA's Site and Design Evaluation System, hosted by the British Stainless Steel Association (BSSA), and it works like this:

• Write down the site facts first: the distance to salt water, to a road that is gritted, the kind of air, and how often the piece is rained on or washed. Nearby sites can differ because of microclimates (IMOA), so do not borrow a neighbour's score.
• Score each of the five sections once, using the rule in each heading: take the highest, pick one, or add all that apply.
• Add the five and read the band: a total of 0 to 2 points to 304 or 304L, 3 to 316 or 316L, 4 to 317L or a more corrosion-resistant stainless steel, and 5 or more to a corrosion expert.
• Score each component separately where the finish, washing or exposure differ across the job. A canopy soffit and a door pull on the same building do not score alike (see the worked examples).
• Look at the flags. A flag means ask a corrosion expert, or test a sample from the site for chlorides, whatever the total says.

IMOA describes the score as a guideline that generates an initial evaluation, not a precise scientific determination, and recommends exposing samples on site to confirm it. It was written from North American practice and is dated 2003, so treat it as a first pass and say so in the file.

The worksheet, section by section

Section 1: environment. Take the highest row that applies. If you are unsure of the pollution level, IMOA says national or local government data can usually be found, and a high reading needs an expert.

Section 2: salt. Score the coastal rows and the de-icing rows separately, then take the higher of the two. If the site has both, ask an expert: IMOA says the combination can be more corrosive than either alone. The distances assume the worst case, a busy road with fast traffic and the wind blowing the road mist towards the site.

Coastal or marine rows

De-icing salt rows

Section 3: weather. Pick one row. Light rain does not wash contaminants off; storms and wind-driven rain can, if they are frequent enough. IMOA gives much of eastern North America and Northern Europe as examples of the −1 row. Whether a given UK site qualifies is a judgement for you and BSSA, and the examples here use 0.

Section 4: design. Add every row that applies, using one finish row only. Smooth means Ra 0.5 micrometres or less. Ask the finisher for the typical Ra of the sample, because the same named finish varies between suppliers (IMOA).

Section 5: washing. Pick one row. The credit is real only if someone has agreed to the washing and owns it.

Flags are IMOA's. “Ask an expert”: a stainless steel corrosion expert with architectural experience should assess the site. “Test a site sample”: some places at that distance have chlorides and some do not, so a sample from the site should be tested.

Three worked examples, and what moves a total

These are illustrations with assumed site facts. They are not assessments of any real site. Example A is a seafront rail within 30 m of the sea.

Raise the finish to Ra 0.3 micrometres or less and the design section drops from 0 to −1, so the total is 3 and the band is 316 or 316L. Take the rain credit as well, if the site earns it, and the total is 2.

Example B is a hotel entrance 20 m from a gritted main road, scored as two components, because the washing and the detail differ.

The soffit scores 8: sheltered, flat, rough and never washed by rain, beside road salt. That is expert territory. The door pull scores 1 on paper, but only because the hotel washes it every month. Without that, it scores 4. The washing credit is the part to check. The de-icing rows also carry IMOA's “test a site sample” flag, so a sample from the entrance should be tested for chlorides whatever the total.

Example C is a capping plate in a clean rural garden, where nothing about the place is harsh.

Clean air does not save a bad detail. A blasted finish, a flat top and an open joint add up to 4 in the countryside. Change the finish to Ra 0.3 to 0.5 micrometres and seal the joint, and the total is 0.

What moves a total. The arithmetic below is ours, from IMOA's rows. IMOA's own list of ways to reduce the score is: boldly expose the piece, choose a smooth finish, use vertical grain, remove horizontal surfaces, remove or seal crevices, design for manual washing, encourage a washing schedule, and add barriers against de-icing road mist.

Where the worksheet stops, and a second guide

Wet interiors. IMOA's system is for exterior stainless steel. We found no scoring method for bathrooms, steam rooms, kitchens or spas in the documents we read, and we have not made one up. For those rooms use the rows in our 304 or 316 sheet and the questions the worksheet rests on:

• Can chlorides reach the surface: salt, bleach, hydrochloric acid, some descalers, chlorinated air?
• Can it be rinsed and wiped dry, and by whom?
• Are there crevices, open joints or flat faces where water and deposits sit?
• Is the finish smooth, and is the grain easy to drain?

An indoor pool hall is a specialist case. Stressed or unwashed parts in the pool air go to an engineer, not to a score.

A second guide. ASSDA, the Australian stainless steel association, gives a plainer rule for tea staining in its Technical FAQ 6, which has no edition date on the page. Within about 5 km of the surf, it says 316 or an equivalent grade should be the minimum; 304 and 430 are likely to tea stain or corrode more severely in marine air. It strongly recommends a roughness below Ra 0.5 micrometres, vertical grain on abraded finishes, and washing the stainless whenever the adjacent window is washed. It also gives a pitting resistance guide: a PRE of about 18 away from the sea, about 24 in marine atmospheres and about 34 in severe marine ones.

IMOA scores 1.6 to 16 km from salt water as 1 point and gives 304 as the most cost-effective choice at a total of 2 or less, so for a site two to five kilometres from the sea the two guides can differ. Where they do, we would start from the stricter one and put the question to BSSA's technical advisory service. Tea staining is brown discolouration from corrosion that is cosmetic, ASSDA notes; IMOA likewise says the corrosion may not affect structural integrity but is aesthetically undesirable.

Writing the result into the specification

• Record the score, with the five section scores, the total and the date, so a later change of detail or cleaning plan can be re-scored.
• Turn the design rows into requirements: a maximum Ra with the direction measured, the grain direction on the drawing, falls so water runs off, crevices sealed or avoided. IMOA says to ask the finisher for typical roughness when samples are requested and to put roughness in the specification.
• Put the washing in the O&M manual, with the frequency, the method and who does it. The washing credit in the score depends on it. Our sheet on cleaning and maintaining metal finishes covers the products.
• Expose a sample. IMOA suggests a sample that simulates the proposed design, exposed on site for four to six months, or about six weeks in severe marine or industrial places. Where chlorides cannot be judged by eye, IMOA says a laboratory can test the surface of an unwashed sample that has had long exposure, collected without removing the deposits.
• Carry it through fabrication. Iron contamination from carbon steel tools, weld heat tint and passivation can undo a good score. Those checks are in the 304 or 316 sheet.

Exposure assessed by [designer] on [date] using the IMOA Site and Design Evaluation System (IMOA ABC 00/03) for [component, location]: environment [0], salt [4], weather [0], design [-1], washing [0]; total [3]. Grade selected: [316L (EN 1.4404)] [after review by a stainless steel corrosion expert / BSSA advisory service where flagged]. The score is re-assessed if the design, location or cleaning arrangement changes.

Stainless steel [316L (EN 1.4404)], finish [EN 10088-2 code, for example 2K], roughness not more than Ra [0.3] micrometres, grain [vertical on vertical faces]. Welds ground to match, not rougher than the surrounding finish. No flat upward faces: falls of [ ] to shed water. Crevices [sealed by weld or sealant / avoided]. As drawing [number, revision].

Washed [at least monthly] with [soap or mild detergent and clean water], rinsed. No cleaners containing chlorides, bleach or hydrochloric acid. Responsibility: [facilities manager]. Entered in the O&M manual at [section]. The washing credit in the site score depends on this schedule.

Before the order is released, [two] samples made to the proposed finish and detail are exposed at [location] for [four to six months / six weeks if severe marine or industrial] and examined by [designer] and [partner]. The order is released on written approval.

Standards and sources

• IMOA: Which Stainless Steel Should Be Specified for Exterior Applications? (ABC 00/03, 2003; via BSSA): the scoring method; all six pages read.
• ASSDA: Technical FAQ 6, Preventing Coastal Corrosion (Tea Staining): no edition date on the page.
• Nickel Institute publication 11 024, Stainless Steels in Architecture, Building and Construction: Guidelines for Corrosion Prevention: named in the IMOA brochure; not read.
• Make Bespoke Studio, TL-MET-01: 304 or 316 stainless steel: the first filter and the fabrication checks.

Questions we are asked

How do I score a site for stainless steel?

Score five sections once each and add them: environment, salt, weather, design and washing. The IMOA method takes the highest row in sections 1 and 2, one row in sections 3 and 5, and every row that applies in section 4. A total of 0 to 2 points to 304 or 304L, 3 to 316 or 316L, 4 to 317L or better, and 5 or more to a corrosion expert.

How far from the sea can I use 304 stainless steel?

IMOA scores 1.6 to 16 km from salt water as 1 point, 30 m to 1.6 km as 3 and under 30 m as 4, and gives 304 as the most cost-effective choice at a total of 2 or less, so it depends on the rest of the score. ASSDA says to use 316 or better within about 5 km of the surf. Where the guides differ, start from the stricter one and ask BSSA.

Does washing mean I can use a lower grade?

On paper, yes: IMOA takes 1 to 3 points off for washing from yearly to monthly. In practice the credit only holds if someone owns the job and does it. If nobody will, score it with no credit.

When do I need a stainless steel corrosion expert?

IMOA says when the total is 5 or more, when the piece is splashed, sprayed or immersed in seawater or de-icing salt, when there is both coastal and de-icing salt, when pollution is high, or in a hot, humid, rainless place with salt or pollution. Also where you are unsure of the evaluation.

Does the score work for bathrooms and kitchens?

Not as it stands. IMOA wrote it for exterior stainless steel, and we found no equivalent scoring method for wet interiors. Use the rows in our 304 or 316 sheet, and treat indoor pool halls as a specialist case.

Is the IMOA score right for UK sites?

It is a 2003 guideline written from North American practice, with distances that apply worldwide. Use it as a first pass, test a site sample where flagged, and ask the BSSA technical advisory service about any UK-specific guidance and about the rain credit.