Stainless steel

Stainless Steel is found on the top nine floors of the Chrysler Building (New York, c. 1930).

Stainless steel (sometimes called inox steel or simply inox) is a steel alloy with no less than 11% chromium content by mass. Due to its high resistance to corrosion, it is also called corrosion-resistant steel (CRES) but only when the type or grade are not detailed. Stainless steel will not corrode or rust easily and it is less likely to stain than carbon steel. Stainless steel differs from carbon steel because of carbon steel’s propensity to rust. Stainless steel has a passive film of chromium oxide that will not cause corrosion, rather it protects from corrosion. Stainless steel can be broken into four distinct categories: martensitic, ferritic, austenitic, and precipitation hardening.

History

In the 1820s, chromium alloy was studied but was thought to not have any corrosion resistance. A British metallurgist announced in 1908that combining chromium with any metal would actually impair the metals ability to repel corrosion. By 1912, Eduard Maurer founded austenitic stainless steel, which contains 18% chromium and 8% nickel. This alloy will harden only through deformation which makes it ideal for deep drawing (forming sheet and strip metal into cup-shapes with a punch-and-die machine). Austenitic is commonly found as formed sheets in architectural projects. In 1911, a German scientist named Christian Dantsizen discovered ferritic stainless steel and used this discovery for the filament in light bulbs. Later, this product was used for turbine blades. Harry Brearly of England discovered martensitic stainless steel in 1913. Martensitic stainless steel is 1% chromium and .35% carbon and was traditionally used for cutlery and munitions. In 1929, Germans W. Kroll and R. Wasmuht introduced precipitation-hardening stainless steel. These men added small portions of titanium, boron, or even beryllium as hardeners. In 1946, an American by the name of E. Wyche patented a form similar to the Germans, but only added titanium as a hardener.

Metallurgy

There are traditionally three steps to properly creating stainless steel. To begin, ordinary steel and iron ore are melted together, causing the oxygen from the iron ore to reacts with carbon to create carbon monoxide. Then, nickel is added to the furnace followed by silicon to reduce the iron oxide. Chromium is the last thing added. The molten metal is poured into its mold and then released from the molds to be forged into sheets and billets. The billets can be rolled into plates, strips, or sheets. When engineering and architecture started to use stainless steel, many new techniques and patterns were developed and patented. Regardless of the finish, all metals underwent a mechanical treatment to smooth the metal and improve its luminosity. A hot rolled, annealed finish (No.1) was the early industry standard until 1934. The Republic Steel and American Rolling Mill companies soon adopted the same standard finishes that are still used today, though the appearance of each differs slightly. A popular treatment for stainless steel is the application of polish by an electrolytic process. In 1947, the Sendzimer cold reduction process was developed with allowed for the cold rolling of thinner sheets and strips of stainless steel. Later developments of stainless steel included the production of gold and bronze plating and super mirror finishes with clear coatings.

Uses and Installation

The use of stainless steel increased exponentially in Great Britain from the early 1900s, but America was slow to employ any of the stainless steel techniques until the mid 1920s. Great Britain began by using stainless steel for things like stoves, cars, and cutlery. Engineering and manufacturing companies in the United States used stainless steel sparingly until The American Architect reported on Britain’s success with the alloy. The Allegheny Steel Company advertised its stainless steel products in 1927, promoting Allegheny Steel for kitchen use and other equipment. Just two years later, Allegheny steels were available for elevators and lobbies as well as for the exterior trim of buildings. Most companies promoted stainless steel for every architectural project, from structural to decorative. Stainless steel was a popular choice for trim, ornamentation, railings, door hardware, light fixtures, counters, signage, and even as decorative art. Stainless steel was promoted for its corrosion resistance primarily, and secondly for its sanitary qualities and modern appearance. In comparison with aluminum, stainless steel had a much heftier price tag but the cost for maintenance was claimed to be much less than aluminum’s. It is not uncommon to see stainless steel in a white metal form being used on Art Deco and streamline buildings. Diner cars were usually made of stainless steel because of its attractive appearance and its impressive corrosion resistance. Two of the most notable architectural projects that employed the use of stainless steel is the Chrysler Building and the Empire State Building. The dome, spire, gargoyles, entrance and doors of the Chrysler Building are made with stainless steel, as is the vertical trim on the Empire State Building. These projects set the stage for stainless steel by showing off its low maintenance and high corrosion resistance. Stainless steel is commonly used in soda fountains, plaques, sculptural elements, as well as storefronts, trim, and front entrances. From the 1940s to the 1950s, the development of the stainless steel curtain wall was developed using thin, lightweight panels that were used to enclose buildings. As the idea of the curtain wall developed, more windows were implemented, leaving stainless steel to act as mullions. World War II brought about the development of the Rigidized Metals Corporation. Rigidized sheets have a rolled pattern to increase strength of the sheet and break up the surface to eliminate distorted surface reflections. A quilted pattern of Rigidized sheets was used extensively in diners in the 1950s. Gutters, roofing and resheeting for existing masonry walls became popular in the 1960s. Certain methods are used for customized and standard applications. Trim is usually applied with nails, rivets, or screws. It can directly attached to steel frames or brackets and the joints can be bronze welded or fusion welded. Expansion joints are a must since thermal expansion is experienced.

Conservation

Though stainless steel is corrosive resistant, it can be damaged in harsh and adverse conditions. It is not unusual for stainless steel to be disfigured by scratches and large dents. It is, however, more durable than most other sheet metals and presents few problems. If the metal does become damaged, it is usually easy to repair.

Identification

• Simple methods

visual examination,spot tests,density

• Scientific methods

xrf,xrd,pixe,libs,electrochemical techniques,metallography

Deterioration

All metals, stainless steel included, are capable of experiencing corrosion. Stainless steel is highly resilient because of its invisible film of chromium oxide. The film does not protect the metal in adverse environments that may contain hydrochloric acids. Also, extreme heat (above 900°F) like welding, even for a short period of time, may cause corrosion. The heat removes chromium, leaving the area susceptible to abnormal corrosion. A blue or orange weld stain around the area is a good signifier that the area has been damaged and may be experiencing corrosion. Pitting is another cause of corrosion. It occurs in stainless steel when dirt build up locks out oxygen from the metal’s surface, restraining the metal’s production of the passive chromium oxide film. Galvanic corrosion occurs when stainless steel is exposed to lead, nickel, copper, graphite, or copper alloys. Generally though, corrosion only occurs in areas of high pollution or where it is exposed to salts or curing mortar. Finally, another damage commonly seen in stainless steel is dents or scratches. Stainless steel in high traffic areas is highly susceptible to these abrasions.

Conservation Techniques - architecture

Keeping stainless steel clean is the best way to prevent unwanted corrosion and maintaining its finish. For routine cleaning, rinsing the surface with warm water and a mild detergent is more than enough to remove loose dirt and pollutants. When cleaning stainless steel, it is important to use a soft cloth, sponge, or plastic pad and always remember to rub in the direction of the grain when possible. Steel wool or brushes should never be used as this may cause pitting. Even soft clothes can cause scratching so rubbing should be minimal. Fresh water should always be run over the stainless steel after wiping to ensure removal of remaining dust or pollutant particles.

Stubborn debris (i.e. grease, oil, fingerprints) can be removed by rinsing with mineral spirits, water, and mild detergent. After washing with this mixture, the metal should be washed with fresh water. When stainless steel has become tarnished by bolts or screws, it is difficult to remove these stains and the only alternative (nitric acid) is not seen as practical.

Removing graffiti can usually be done in the same manner as dirt of fingerprints. For water-soluble paints, a mild detergent and fresh water will likely remove the debris. For stubborn oil-based paints, de-ionized water and mineral spirits generally prove stout enough. Paste wax is the ideal way to remove pencil marks. All items should be rinsed thoroughly with fresh water.

When trying to clean weld stains on stainless steel, the most aggressive approach is to use a mild paste abrasive and clean water. Minimizing weld stains is possible by inserting a thin aluminum piece between the welding tip and the stainless steel on the side to be kept clean. When stainless steel has corrosion, it is capable of being remedied by warm water, detergent, and a plastic pad. When the corrosion is more stubborn, grinding or sandblasting can resolve the issue. Using this technique to clean stainless steel may require a refinishing on the surface with a fine abrasive, which will make it match the original piece. If all other techniques fail, new pieces of stainless steel can be spliced in by welding and can be made to look almost identical to the existing materials. Most scratches will serve as no more than a visual annoyance. If several scratches appear on the surface, the location and amount of material used should be evaluated in order to prevent further damage. Also, the severity of the scratch determines the level of treatment that will be required to repair it. Minor scratches may only need a rubbing compound that contains a light abrasive, where heavier scratches may require a grinder to even the base of the finish. If the scratch is deep, it can be filled with a weld and finished to look like the existing materials.

Conservation Techniques - historical objects

Cleaning

• Mechanical
• Chemical

Mixed organic acids

47,5 gms citric acid

47,5 gms sodium gluconate

4,9 gms tartaric acid

1-10 litres water

Immerse object in cold or hot solution,then rinse well.

According to:

USPT 4,264,418

• Electrochemical
• Ultrasonic
• Laser

Replacement

Existing finishes is the main concern when replacing stainless steel. It is imperative to match field examples to industry standards before ordering replacement pieces. Common finishes for stainless steel include No. 2B (bright,cold-rolled), No. 4 (polished finish), No.6 (dull satin), and the mirror finish known as No. 8. Experimentation with abrasives and finishings may be required to match pieces. A test piece is strongly suggested in the field before repairs are performed. When creating a piece out of stainless steel, it is important to leave room for shrinkage. Also, just because the two finishes are labeled the same thing, this usually does not mean that they will be identical. The finishing processes have evolved over the year and it is unlikely that something from the 1940s will match perfectly to something done in the 1990s.

References

• Score, Robert, and Irene J. Cohen. "Stainless Steel." Twentieth-century Building Materials: History and Conservation. By Thomas C. Jester. New York: McGraw-Hill, 1995. 64-70. Print.

Further reading

Selwyn,L. Metals and Corrosion .A Handbook for Conservation Professional,Ottawa 2004.

External Links

Cleaning Methods for Stainless Steel

Examples

The Gateway Arch .(St. Louis, Missouri, c.1965).   The Unisphere (Queens, New York, c. 1965). The Unisphere (Queens, New York, c. 1965).   Stainless steel sink