Metallurgy of Stainless Steel and Cobalt – Chromium alloys

CLINICAL IMPLICATIONS

 

Mollenhauer has commended on the general performance of these wires. They are difficult to bend but interestingly when they are removed from the patient’s mouth several months later they are still stiffer than S.S. wires which tend to soften in the mouth. The PS wires are (0.020) 3 times stiffer than the SS wires and are excellent base arches. The supreme wires are a valuable adjunct to stage 2 producing very light forces.

Braided and twisted wires

Very small diameter stainless steel wires can be braided or twisted together by the manufacturer to form larger wires for clinical orthodontics. The separate strands may be as small as 0.178mm and their final cross-sectional dimension is bet 0.406mm and 0.635mm. Available in round and rectangular shape.

 

They are able to sustain large elastic deflections in bending. These wires apply low forces for a given deflection when compared with solid stainless steel wire.

 

TYPES AND MECHANISMS OF CORROSION

 

Among today’s archwires, the nickel-containing “memory” wires and stainless steel wires are more likely to be corroded than the cobalt-containing Elgiloy and beta-titanium types

The corrosion resistance of orthodontic grade steel is directly proportional to its chromium content and inversely proportional to its carbon content. No steel is truly “stainless”, but stainless steel protects itself by forming a protective chromium oxide layer from oxidation products of the alloy.

Any conditions such as extreme pH, physical abrasion, galvanism, or reactive extraneous ions that disrupt the chromium oxide layer will render the steel vulnerable to corrosion. Instruments made of carbon or 400 series steel are more susceptible than those made of 300 series steel.

 

(JCO Volume 1987 May, Sterilization in Orthodontics – SHANON T. KIRCHHOFF, DDS, ROBERT K. SEKIJIMA, DDS, MARK I. MASUNAGA, DDS, CYRUS M. ALIZADEH,)

 

The most notable effects of corrosion are the loss of metal weight and the weakening of mechanical properties.

 

Corrosion occurs in several ways:

 

Uniform corrosion.

The metal is attacked evenly and throughout, and its mechanical properties diminish proportionately with weight loss. This type of corrosion is rarely seen in orthodontic attachments, since they are not evenly exposed to corrosion agents.

 

Localized or pitting corrosion.

The most common form of corrosion in orthodontic attachments affects the mechanical properties or aspect much more than could be inferred from the weight loss. It happens when the attachment is made of several parts, is improperly treated, or contains impurities.

 

Crevice corrosion.

This occurs when the attachment is in contact with plastic materials — an adhesive, an acrylic prosthesis, or an elastic — and is common in orthodontics .

 

Microbiologically induced corrosion.

Undetected until recently in dentistry, but widespread in industry, microbial attack is directed mainly against the bracket base and occurs especially in non-aerated, sensitized areas such as the junction between mesh and foil. Several types of microorganisms attack the iron in stainless steel. The first type, which generate corrosive agents, are either sulfate reducers, such as Desulfovibrio desulfuricans or Desulfotomaculum; sulfur oxidizers, such as Thiobacillus ferroxidans, Beggiatoa, or Thiotrix; or slime formers, such as Aerobacter or Flavobacterium. Other microbes, such as Sphaerotilus, Hyphomicrobium, and especially Gallionela, actually consume iron.

 

 

Intergranular corrosion.

In this form of dissolution, chromium carbide separates at the steel grain boundaries and consumes part of the protective chromium oxide layer. The chromium carbide film is then readily attacked and dissolved, with catastrophic consequences. This separation can start at temperatures as low as 350°C,11 which means that microstructural weakening can occur during brazing, welding, and cold working as well as thermal reconditioning.

 

Galvanic corrosion

When in contact with an electrolyte such as saliva, metals corrode by a complex electrochemical process of oxidation and dissolution known as galvanic corrosion. In oxidation, metal atoms lose electrons and thus become positive (cations) and soluble. The remaining metal is electrically charged and acts as an anode. The generation of an electric cell is simple when differing metals are involved, but it can also occur within a single metal. Atoms at the grain boundaries dissolve faster than those within the grain, and impurities within rough surfaces or irregularities can also alter the corrosion resistance of a metal. A portion of a metal that is subjected to cold working or stress becomes more anodic and thus dissolves faster than the rest of the metal. Galvanic corrosion usually follows a specific course in the case of stainless steel, because the chromium oxide protective layer is not resistant to chloride ions and cannot be regenerated in the absence of oxygen.

brazing materials can be more or less noble than stainless steel, causing either the dissolution of less noble silver brazing or an attack on the steel .Soldered and welded joints will produce a galvanic couple and thus corrosion. Austenitic stainless steels are susceptible to attack by solutions containing chlorine.

BIOCOMPATIBILITY

 

Diseases Caused by Mixed Metals in the Mouth

Leaking of heavy metals (nickel, cobalt, and chromium) as a result of corrosion can also cause various reactions.4-7 About 9% of women and 1% of men are allergic to nickel, and cobalt allergies affect about 1% of the population, especially women.8 The incidence of nickel allergies is even higher in some countries, with the 10-to20-year age group showing the highest frequency of first-time symptoms. When such allergies are suspected, the patient can be tested for sensitivity with a nickel patch; however, the ADA warned in 1984 that even these patches can cause irritation. Components of soldering and brazing (zinc, copper, and especially cadmium) can further contribute to such reactions.

   Tooth staining is another phenomenon that accompanies bracket corrosion. A study showed

that one cause of staining was the diffusion of metal ions into the resin, which in turn

remained ingrained in the enamel.

  ( JCO Volume 1995 Jan- Attachment Corrosion and Its Testing  CLAUDE G. MATASA, PHD, D)

Nickel Hypersensitivity

 

Symptoms of toxicity : Rash, allergy and lung disorders. Ear infection and tinnitus.           Nickel is one of the most common causes of allergic contact dermatitis
and produces more allergic reactions than all other metals combined. Stainless steel contains Nickel and it can sometimes produce allergic reactions in the
oral cavity.

 

Justin K. Bass and Howard Fine conducted a study on 29 patients ranging from 12-48 years of age. The objectives of the study was

 

(1)      To determine if standard orthodontic therapy can sensitize patients to Nickel.

(2)      To assess gingival response to orthodontic appliances containing Nickel on patients who are Nickel sensitive before treatment.

 

The results obtained were:

 

(1)         Prevalence of Nickel allergy is higher in females than in males.

(2)         Nickel containing orthodontic appliances had little or no effect on the gingival and oral health of the patient.

Orthodontic treatment may induce Nickel sensitivity.

 

Chromium
Symptoms of toxicity: Depression and emotional disorders. Chromium is one of the major components of insulin. Chromium deficiency causes high blood sugar and diabetes.

 

Cobalt

Symptoms of toxicity: Cobalt is carcinogenic and causes tumor. Sedation of the limbic nervous system, paralysis on the left side of the body, particularly the neck and shoulder. Nervous twitch, loss of control of body movement, clumsiness, chronic fatigue. Embolism and stroke, blockage in the carotid artery on the left. TMJ. Pins and needles, numbness and tingling in the extremities. Pernicious anemia. Blood disorders and liver malfunction. (Cobalt is a natural component of B12, therefore cobalt poisoning causes B12 deficiency.

NICKEL FREE STAINLESS STEEL

In recent years a very new kind of nearly nickel free austenitic s s was developed and introduced to the market. The steel is alloyed with 15-18% chromium, 3-4% molybdenum, 10-14% manganese, and about 0.9%nitrogen to compensate for nickel. This steel has got high corrosion resistance. The low nickel concentration results in the reduction of nickel allergy potential. Orthodontic wires under the name Manzanium (Scheu), or Noninium (Dentaurum) are already in the market. Unfortunately the melting and forming of this steel is very costly.

SOLDER FOR STAINLESS STEEL

 

Stainless steel wire should not be heated to too high a temp to prevent carbide precipitation and to prevent an excessive softening of the wire. So a low temp soldering is preferred rather than the high temp gold solders. So silver solders are generally preferred.

Silver solders are alloys of silver, cu, and zinc to which tin and indium are added to lower the fusion temp and improve solderability. The soldering temp for orthodontic silver solder are in the range of 620-650oc.

SOLDERING FLUXES

The flux used for soldering stainless steel contains fluoride to dissolve the passivating film formed by the chromium. The solder does not wet the metal when such a film is present. Potassium Fluoride is one of the active chemicals in this respect.

 

WELDING

Flat structures such as bands and brackets are usually joined by welding. The electric spot welding apparatus produces a large electric current that is forced to flow through a limited area on the overlapped materials that are to be welded. The resistance of the metal produces intense localized heating and fusion of overlapped metals. The strength of the welded joint is decreased with increased recrystallization of the wrought structure. The strength increases with increase in weld area. The weld area becomes susceptible to corrosion primarily because of chromium carbide precipitation and consequent loss of passivation. This is known as weld decay.

 

 

 ADVANTAGE OF STAINLESS STEEL WIRES

 

1)            Lowest cost of the wire alloys

2)            Proven biocompatibility for fabrication into orthodontic appliances.

3)            Can be soldered and welded although welded joints may require solder refinement.

 

DISADVANTAGES

 

1)            High force delivery

2)            Relatively low springback in bending compared to b- titanium and Ni-Ti alloys.

3)            Can be susceptible to intergranular corrosion after heating to temperatures required for joining.