Stainless steel is a generic term for a family of corrosion resistant alloy steels containing 10.5% or more chromium.

All stainless steels have a high resistance to corrosion. This resistance to attack is due to the naturally occurring chromium-rich oxide film formed on the surface of the steel. Although extremely thin, this invisible, inert film is tightly adherent to the metal and extremely protective in a wide range of corrosive media. The film is rapidly self repairing in the presence of oxygen, and damage by abrasion, cutting or machining is quickly repaired.

Fig. 1 - In any normal oxidising environment a protective coating of passive chromium rich oxide film is automatically formed on stainless steel.

Fig. 2 - When scratched, damaged or machined this protective film is denuded exposing the steel to the atmosphere.

Fig. 3 - The protective coating is quickly restored through the rapid self-repairing quality of the chromium rich film.

304: the base product

UNS S30400 (grade 304) is the greatest stainless success story. It accounts for more than 50% of all stainless steel produced, represents between 50 and 60% of Australia's consumption of stainless materials and finds applications in almost every industry.

Corrosion resistance

Grade 304 has excellent corrosion resistance in a wide range of media. It resists ordinary rusting in most architectural applications. It is also resistant to most food processing environments, can be readily cleaned, and resists organic chemicals, dye stuffs and a wide variety of inorganic chemicals. In warm chloride environments, 304 is subject to pitting and crevice corrosion and to stress corrosion cracking when subjected to tensile stresses beyond about 50oC. However, it can be successful in warm chloride environments where exposure is intermittent and cleaning is a regular event (such as saucepans and some yacht fittings).

Applications

Alternative grades to 304 should be considered in certain environments and applications, including marine conditions, environments with temperatures above 50-60oC and with chlorides present, and applications requiring heavy section welding, substantial machining, high strength or hardness, or strip with very high cold-rolled strength.

However, typical applications for 304 include holloware, architecture, food and beverage processing, equipment and utensils, commercial and domestic kitchen construction, sinks, and plant for chemical, petrochemical, mineral processing and other industries.

With this breadth of application, grade 304 has become a fundamental alloy in modern industry and is certainly worth committing to your materials knowledge base.

304 is not the only stainless steel and is not appropriate in every application. However, an understanding of the attributes of 304 provides an excellent base for comparing members of the austenitic family of stainless steels and a practical base for determining the appropriateness of stainless steel in a given application.

You already have substantial experience of 304 and its properties on which to draw. Chances are some of your cutlery (look for the telltale 18/8 or 18/10 designation), your saucepans, your sink or, even, the shutter on your floppy disk are 304 stainless.

316: the first step up

If a job requires greater corrosion resistance than grade 304 can provide, grade 316 is the 'next step up'. Grade 316 has virtually the same mechanical, physical and fabrication characteristics as 304 with better corrosion resistance, particularly to pitting corrosion in chloride environments.

Grade 316 (UNS S31600) is the second most popular grade in the stainless steel family. It accounts for about 20% of all stainless steel produced.

Corrosion resistance

Grade 316 has excellent corrosion resistance in a wide range of media. Its main advantage over grade 304 is its increased ability to resist pitting and crevice corrosion in warm chloride environments. It resists ordinary rusting in virtually all architectural applications, and is often chosen for more aggressive environments such as sea-front buildings and fittings on wharves and piers. It is also resistant to most food processing environments, can be readily cleaned, and resists organic chemicals, dye stuffs and a wide variety of inorganic chemicals.

In hot chloride environments, grade 316 is subject to pitting and crevice corrosion and to stress corrosion cracking when subjected to tensile stresses beyond about 50oC. In these severe environments duplex grades such as 2205 (UNS S31803) or higher alloy austenitic grades including 6% molybdenum (UNS S31254) grades are more appropriate choices.

The corrosion resistances of the high and low carbon versions of 316 (316L and 316H) are the same as standard 316. They are mostly chosen to give better resistance to sensitisation in welding (316L) or for superior high temperature strength (316H).

Applications

Typical applications for 316 include boat fittings and structural members; architectural components particularly in marine, polluted or industrial environments; food and beverage processing equipment; hot water systems; and plant for chemical, petrochemical, mineral processing, photographic and other industries.

Although 316 is often described as the 'marine grade', it is also seen as the first step up from the basic 304 grade.

Cleaning your Indoor Stainless Steel

Quick and Easy Tips for Keeping That Shine

Retaining a sparkling finish on stainless steel surfaces is just a matter of a few simple steps. And you don't need expensive products or special equipment - ordinary household cleaners are usually all that's required.

You just need to bear in mind a few easy DOs and DON'Ts...

It'll come out in the wash

Stainless steel looks best if it's cleaned regularly with plenty of water. Drying afterwards makes sure streaky marks aren't left behind.

Remember that simply wiping with a damp cloth is not as effective as it can smear dirt without removing it.

Routine cleaning prevents any stubborn stains building up.

So what will you need?

You don't need any fancy equipment. For day to day cleaning, plenty of water, some mild detergent and a cloth or soft brush will do the job. You can use a 1% ammonia solution but don't use bleach ? it's just too easy to make the solution too strong and too hard to rinse it properly afterwards.

After washing, rinse in clean water and wipe the surface dry with a soft absorbent cloth. On brushed stainless steel, follow the direction of the polish for best results. An excellent cloth to use is 3M's Scotch-Brite high performance cleaning cloth. Watch out for scratches!

The important thing to remember is that stainless steel can be scratched by careless handling or aggressive scrubbing. Just like you would take care of a polished timber finish, avoid dragging rough items across the surface and be aware that grit trapped under other objects can be the culprit. Avoid bad chemistry

Stainless steel may discolour if left in contact with salts or acids for extended periods. Also avoid leaving carbon steel items in contact with stainless steel, particularly if wet.

How to handle the tough customers

Sometimes you need a tougher approach. Here's how to get rid of the most common offenders:

Fingerprints, oil & grease marks

If a mild detergent or dishwashing detergent doesn't shift unsightly fingermarks, get rid of them with a bit of glass cleaner on a soft cloth. You can also use a small amount of alcohol, methylated spirits, acetone or mineral turpentine. Then rinse with clean water and dry.

You can give longer protection to high traffic areas by lightly rubbing with olive oil or baby oil followed by a polish and shine using a soft cloth.

Tea & coffee stains

Discolouration from tea and coffee stains can be removed by soaking in a solution of boiling water and baking powder. Remember to rinse well and wipe dry.

Sticky labels

Remove sticky labels as soon as possible. Gentle heat from a hair dryer or a glue gun generally softens the glue for easy removal, or you can warm stainless steel pots and pans in the oven before peeling off the labels. Eucalyptus oil based cleaners (or eucalyptus oil on its own) often work well to remove adhesives.

Ensure you don't leave any glue on the surface ? it could trap dirt or break down and cause staining.

Rust marks and 'tea staining'

Apply cream cleanser with a soft damp cloth and rub gently.

If the mark still won't shift, it might be necessary to use a proprietary stainless steel cleaner. These are usually based on dangerous chemicals (such as phosphoric acid) and must be handled with care according to the manufacturer's directions.

These products do not usuall require any scrubbibg or mechanical cleaning. It is simply a matter of wiping the product on the surface and leaving for a recomented period of time to disolve the stain.

After cleaning it is important to neutralise the acid with a 1% ammonia or baking powder solution or wash with soapy water, rinse with clean water and wipe dry. As an added bonus every time this is done the surface will be mildly passivated and will become more resistant against future tea staining.

Paint

Apply paint stripper, taking care to follow the safety instructions. You may need to use a nylon brush or scouring pad, but avoid metal scrapers at all costs - they will damage the surface.

Hard water scale

Heavy limescale from hard water can be loosened by soaking in a hot water and 25% vinegar solution. Rinse well with a solution of baking powder or 1% ammonia and then with clean water. Always wipe dry.

Cement and mortar

Cement and mortar splashes should be washed off before they set. Mild acids such as vinegar may be needed but not those using chloride rich chemicals. Never use brick cleaning liquids which contain hydrochloric acid. Be very careful that loosened particles don't scratch the steel surface.

Don't go against the grain

Always rub stainless steel in the same direction as the grain. Rubbing against the grain will spoil the finish and stainless will lose its shine. Worse, rubbing against the grain can damage the surface by creating microscopic crevices where dirt can collect. This can lead to corrosion spots.

Fortunately, it's usually easy to tell which is the right direction. You need to watch out for items like round handrails, which are often polished around their circumference when they're manufactured, rather than up and down the length of the tube.

If you have to scrub a stain to remove it, make sure you use a clean nylon scourer or a cloth with chalk-based cream cleaner. But test an inconspicuous area first as you could end up with a bright polished spot which doesn't match the rest of the surface.

NEVER EVER use steel wool (wire wool) to clean stainless steel.

It is usually made of carbon steel and any fragments left behind will rust onto the stainless steel surface. Using any kind of scourer which has previously been used on ordinary (carbon) steel is also a no-no for the same reason.

Stainless steel wool scouring pads are available for heavy duty work, like removing burnt food from stainless steel saucepans. These will scratch the stainless steel surface, but won't leave fragments to go rusty.

Preventing Coastal Corrosion (Tea Staining)

When used properly, stainless steel enjoys a strong and enduring reputation for visual appeal and structural integrity in a wide range of applications and environments.

Incorrect specification, fabrication or subsequent handling in coastal environments may cause stainless steel to stain or discolour, impairing the overall look.

This brown discolouration, or tea staining, has been identified in coastal applications in Australia and overseas.

Tea staining does not affect the structural integrity or the longevity of the material and it can be controlled.

Where does it occur?

Tea staining occurs most commonly within about five kilometres of the surf or a few hundred metres from a sheltered bay. It becomes progressively worse closer to the marine source.

However, wind exposure, industrial pollution levels and higher temperatures can create environments where tea staining might occur 20 kilometres from the sea. These same factors also increase corrosion rates of alternative metals.

Plan To Get The Desired Result

Marine environments are the most aggressive for all building materials. Stainless steel's very slow corrosion rate in marine environments means that installations are likely to remain structurally sound for decades.

It must be recognised however, that keeping a pristine surface finish requires understanding, additional effort and usually extra costs. Determine your expectation of the structure and plan ahead to achieve and maintain the intended result.

Design, Fabrication and Handling

Poor design and fabrication can lead to tea staining or more serious corrosion of stainless steels.

Good designers will avoid crevices, such as intermittent welds, and areas where water can collect. Competent stainless steel fabricators will avoid carbon steel contamination.

Appropriate Grade Selection

Exposure of a particular grade of stainless steel to a more aggressive environment than it can resist will contribute to tea staining.

Grade 316 should be selected as a minimum within five kilometres of a surf coast. For very aggressive applications (eg. splashed by sea water), unwashed areas, rough surfaces or those applications where no staining can be tolerated, higher grades of stainless steel such as 2205 or a 'super' grade may be required.

The less alloyed, and cheaper grades, (such as 304 or 430) will probably become tea stained or could suffer more severe corrosion.

Specify and Insist on a Smooth Surface Finish

Rough surface finishes promote tea staining, especially with the mechanically polished No.4 finish: the smoother the surface, the better. Smoother surface finishes stay cleaner between washes and don't have deep surface grooves where chlorides and other contaminants can collect and concentrate.

For a mechanically polished surface, a surface roughness (Ra) of less than 0.5 micrometres is strongly recommended. A No.4 finish will not always be within this limit unless specified; it is recommended that all No.4 Typically, refinishing operations using 320 grit abrasives will achieve a finish better than 0.5 micrometres Ra.

All the finishes can be made more corrosion resistant for use near the ocean by passivating with acid to remove surface contaminants. This is especially beneficial for No.4 and other abraded finishes. Apart from possible clouding of mirror polished materials, passivation has no effect on the appearance of the surface before it goes into service.

Do Not Use Hydrochloric Acid

Hydrochloric acid is sometimes used to clean cement or mortar residues. This acid should never be used on or near stainless steel - it will frost the surface and usually causes pitting.

Wash Regularly

In coastal environments even smooth stainless steel finishes may tea stain if not washed regularly to remove salt. As a rule of thumb, it should be washed as regularly as the windows.

Perform regular maintenance

Washing removes contaminants (such as salt) that can cause corrosion and is necessary to avoid tea staining. Rain washing the surface is helpful in reducing tea staining, so design the job to take advantage of the rain, but ensure good drainage.

Stipulate that the stainless steel also be washed when cleaning of the surrounding area takes place. For best results wash with soap or mild detergent and warm water followed by rinsing with clean cold water. The appearance of the surface can be improved further if the washed surface is wiped dry. If routine cleaning of the surrounding area does not take place, washing frequency for the stainless steel is recommended.