• Stainless Steel in Architecture

Cleaning and Maintenance

The architect’s responsibility

The architect can be the most important contributor to a building’s low-cost maintenance by careful attention to some basic design considerations.

  • The structure should be, as far as possible, self-cleaning by the natural elements.
  • The architect should minimise horizontal components that can collect dirt. This dirt, when washed off by rain. may cause uneven streaking of the areas below.
  • Designs that concentrate or directionalise the flow of rainwater should be avoided. An overhang can be protected beyond any lower one to avoid splatter or concentration of dirt-carrying water.
  • Sheltered areas, such as canopies or soffits, should be designed so they can be readily cleaned, particularly in low, street-side locations.
  • Joint designs that minimise dirt accumulation should be used.
  • The possibility of staining of the stainless steel by run off from other materials, e.g. rust from carbon steel, copper and aluminium, including hidden clips or fasteners, must be avoided.
  • Grooves, recesses, and excessively complex contours, which hamper the regular easy cleaning associated with stainless steel, should be avoided.

Fabrication shop practice

Stainless steel is protected by a natural clear oxide film, which forms on its surface when the metal is exposed to air. Despite this protection and the inherent strength and hardness of stainless steel, its surface can be damaged during fabrication and shipment if proper precautions are disregarded.

In the fabricating shop, good housekeeping is particularly important.

The following are points that should be watched during fabrication:

1.        Prevention of Contamination

Isolation of the stainless steel processing area. All work should be done in an area set aside for that material and isolated from other metals. This is to avoid the possibility of the pick up of carbon steel and other contaminants by the stainless steel surface.

  • All wire brushes must be made from stainless steel.
  • All tools used to grind or polish stainless steel must be appropriately suited to and dedicated to stainless steel and not used for other metals.
  • Where guillotines or shear blades are used they should be thoroughly cleaned to remove any carbon steel chips or shavings from the area prior to working with stainless steel. To minimise pick up from the blade, if possible the guillotines or shear blades should be kept in a dedicated area and made from hard chrome plated steel or high carbon, high chrome steel.
  • Rolls, forming mandrels, press brakes, etc., should only be used when they have been thoroughly cleaned to remove any carbon steel particles. If possible, the tool should he kept in a dedicated area and made from hard chrome plated tool steel, high carbon-high chrome steel or from one of the aluminium bronze tool materials.

2.        Local Heating

Because of the combination of low thermal conductivity and high thermal expansion, care must be taken with austenitic stainless steels to minimise any local heating during welding, grinding, polishing and buffing that could cause warping and / or buckling in the areas being treated in this way.

3.        Thermal Expansion

The austenitic stainless steels have a higher coefficient of thermal expansion than carbon steels. It is therefore necessary to allow for expansion joints in panels and particularly long runs of metal such as rainwater guttering

4.        Work Hardening

The austenitic stainless steels have a high propensity to work hardening. The progressive forming stages involved in the fabrication must consider the additional allowance in over bending for increased spring back.

5.        Welding

Care must be taken prior to welding to remove any lubricants, paint or other carbon containing material that could contaminate the weld.
When the steel is welded, all heat marking, slag or weld spatter must be removed. “Pickling Paste” is commercially available, and is often the most convenient way to remove the weld scale discolouration. Stray arc strikes must be avoided.

Contamination from copper, lead or zinc or alloys containing these metals can also cause cracking in the heat affected zone. If stainless steel is welded to galvanised steel the adjacent galvanised layer must first be totally removed from the region to be welded. This cleared region would generally then require protection such as by an organic coating after welding.

6.        Fasteners

Fasteners should be made from the same material or one that is more corrosion resistant than the base material. Under no circumstances should free machining stainless steel fasteners made from Grades 303 or 416 be used in stainless steel fabrications where outdoor corrosion resistance is required; certainly not in seaside environments.

7.        Sheet Selection – for matching finish control

In large jobs, care should be taken to select sheets produced by the manufacturer from the one batch and use them sequentially in the order in which they were cut to ensure colour and finish matching between following sheets. This is particularly important for grit polished and embossed sheets, which may further need to be aligned with common orientation to achieve this.

8.        Passivation and Pickling

Final shop operations usually include cleaning of all surfaces with suitable solvents to remove all lubricants used in cutting and forming, as well as contaminants resulting from welding, soldering, grinding and finishing. This may call for “passivation”, a process in which the product is dipped in, or swabbed with, an appropriate acid solution followed by a thorough water rinse to remove all traces of the passivating fluid or paste. Not only does passivation dissolve contaminants but it also increases corrosion resistance. Dark weld scale must be removed by “pickling”, as described below. Note that some pickling conditions will change the appearance of the steel surface. Further details are given in ASTM A380, which is an excellent source of information on all aspects of cleaning stainless steels.

Passive treatments

  • Grades with at least 16% chromium (except free machining grade such as 303): 20-50% nitric acid, at room temperature to 40°C for 30-60 minutes.
  • Grades with less than 16% chromium (except free machining grades such as 416): 20-50% nitric acid, at room temperature to 40°C for 60 minutes.
  • Free machining grades such as 303, 416 and 430F: 20-50% nitric acid + 2-6% sodium dichromate, at room temperature to 50°C for 25-40 minutes.

Pickling treatments

  • All stainless steels (except free machining grades): 8-11% sulphuric acid, at 65 to 80°C for 5-45 minutes. This loosens tight scale before other pickling or passivation treatments.
  • Grades with at least 16% chromium (except free machining grades): 15-25% nitric acid + 1-8% hydrofluoric acid, at 20 to 60°C for 5-30 minutes.
  • Free machining grades and grades with less than 16% chromium such as 303, 410 and 416: 10-15% nitric acid + 0.5 – 1.5% hydrofluoric acid, at 20 to 60°C for 5-30 minutes.

A convenient method of pickling is using proprietary “pickling paste”. This has acids as the second treatment above, but suspended in paste. This method is particularly convenient for spot treatment of weld scale or spatter, and is also applicable to vertical and overhead surfaces.

Safety Precautions – Pickling & Passivation

Acids should only be used for on-site cleaning when other methods have proven to be unsatisfactory. Rubber gloves and safety glasses must be used, and care taken to see that acid cleaners are not spilt over adjacent areas. It is most important that all residues are neutralised and thoroughly flushed away to a suitable waste system. Always dilute acid residues by adding acid to water, not water to acid. Use acid-resistant containers, such as glass or plastics.

Solvents should not be used in confined spaces. Smoking must be avoided when using solvents.

Chlorides are present in many cleaning agents. If a cleaner containing chlorides, bleaches or hypochlorite’s is used it must be promptly and thoroughly cleaned off afterwards.

Fabrication considerations for coloured stainless steel

Normal bending and angular bending that is used for processing building and elevator materials can generally be carried out.

Colour Change after Bending
Bending results in some stretching of the grain along bend lines, however the overall appearance of the coloured stainless steel sheet is little affected.

Coloured stainless steel can be processed in the same way as ordinary stainless steel sheet, there is however colour change due to compressive strain. Trials should be conducted to ensure acceptable results are obtained.

Coloured stainless if welded is likely to discolour along the weld zones and must be cleaned in the same manner as for non-coloured steel.

Accordingly, to maintain aesthetic appeal it is advisable that welds not be in exposed positions. Welded areas may be strategically placed to conceal them at joints or such locations.

For a comprehensive guide to key construction/fabrication considerations of coloured stainless steel, contact Atlas Steels direct and ask for our Coloured Stainless Steel Construction Manual.

Surface protection film

Where stainless steel is delivered with a protective plastic film, the purchaser should be advised that the film should be removed as soon as possible to avoid ultra-violet light degradation of the adhesive on long term warehouse storage as this can make it difficult to remove the film. Outdoor storage of plastic coated product must be particularly avoided.

In addition, degradation of the film can result in corrosive attack of the stainless steel.

The customer should also be advised to remove all traces of the adhesive that may remain on the metal when the film is removed. The region under this residual adhesive could act as a potential source for future corrosion, by collecting dirt. A solvent can be used to remove the adhesive remnants.

Cleaning and maintenance at the construction site
Often, absorptive wrappings such as interleaving paper, cardboard, and other materials are used to protect stainless steel during on-site storage. Such material should not be allowed to become wet because water-soaked paper may discolour stainless steel. Tarpaulins or plastic sheeting must be used to protect the stainless steel. This is particularly important on construction jobs where dirt, dust, carbon steel particles from grinding or welding, etc. in the presence of moisture may cause discolouration. Indoor storage is preferred. Note previous comments regarding avoidance of long-term storage with plastic coating.

Any drainage from concrete or mortar containing chlorides must be immediately removed. This is particularly true when cleaning masonry with strong acid cleaners.

Protective coatings such as adhesive paper or plastic when stripped from the stainless steel can leave small amounts or a very thin coat of adhesive on the metal surface. This facilitates the adherence of airborne dirt particles, and the removal of the adhesive residue is important to maintain good overall appearance. A thorough initial cleaning is therefore required. The recommended practice is as follows:

  1. The surface is pre-cleaned using a slow evaporating solvent system that is compatible with water and that exhibits low toxicity. It is wiped or brushed using a long-fibred nylon brush and light strokes. It is advisable to work on one reasonably small area at a time, i.e. lmtr x lmtr;
  2. The surface is cleaned with detergent by wiping or brushing as in step 1;
  3. It is rinsed with clean ambient-temperature water until all detergent residue is removed; and
  4. It is dried in ambient air; the use of a squeegee is recommended.

This initial cleaning is to be followed at regular intervals (on curtain walls when the glass is washed) by normal cleaning:

  • The surface is rinsed with water to remove as much soil as possible;
  • Soap or liquid detergent or 5% ammonia solution in water is applied;
  • It is rinsed well with water; and
  • Water is removed, ensuring that all strokes are in the same direction, preferably top to bottom and overlapping, and the surface is then allowed to dry.

On-going maintenance of stainless steels in service

The attractive and hygienic surface appearance of stainless steel products cannot be regarded as completely maintenance free. All grades and finishes of stainless steel may in fact stain, discolour or attain an adhering layer of grime in normal service. To achieve maximum corrosion resistance the surface of the stainless steel must be kept clean. Provided the grade, condition and surface finish were correctly selected for the particular service environment, fabrication and installation procedures were correct and that cleaning schedules are carried out regularly, good performance and long service life will be achieved.

Advice is often sought concerning the frequency of cleaning of products made of stainless steel, and the answer is quite simply “clean the metal when it is dirty to restore its original appearance”. This may vary from one to four times a year for external applications or it may be once a day for an item in hygienic or aggressive situations. In many applications the cleaning frequency is after each use. Frequency and cost of cleaning is lower with stainless steel than with many other materials and this will often out-weigh the higher acquisition cost.