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Titanium

Titanium is an element (symbol Ti) like gold, silver and platinum. Its atomic number is 22, with atomic weight of 47.9. It is a silvery white non ferrous metal with the highest strength to weight ratio of any known element. For this reason titanium is the favored material in the aerospace industry. 85% of the structural components in the Space Shuttle are made of titanium. Titanium has been embraced by the medical world for its superior bio-compatibility and is the preferred material for surgical instruments. Titanium is inert and therefore completely corrosion resistant. It does not react to salt water, sunlight, microbes, or body/common chemistry.

A stable, substantially inert oxide film provides the material with outstanding resistance to corrosion in a wide range of aggressive media. Whenever fresh titanium is exposed to the atmosphere or to any environment containing oxygen, it immediately acquires a thin tenacious film of oxide. It is the presence of this surface film that confers on the material its excellent corrosion resistance. Provided that sufficient oxygen is present, the film is self healing and re-forms almost at once if mechanically damaged.

This is the premier metal for our tributes. We have developed a technique to image it, but it is expensive. The piece pictured above is a photo of a small titanium tribute. And just to show people that the metal is actually etched we removed the surface coating on a test sample, the train piece pictured below. You can still see the image quite well even though there is no surface color providing contrast. Dark areas are shadows caused by light refraction on the smooth vs. etched surface.

Oxidation

There is good oxidation and bad oxidation. Oxidation is the interaction between oxygen molecules and the different substances they may contact. Not all oxygen interaction destroys metal. In the case of iron it causes a slow burn which manifests itself as rust. That eventually destroys the metal. In the case of copper, oxidation forms a protective, greenish to black patina of copper oxide compounds. When bronze oxidizes it get darker. Some people like it, others do not. The metal is not harmed. Many manufacturers coat the bronze with a lacquer or other organic coating to retard oxidation. You must keep re-coating the bronze with a barrier film or wax it - like they do bronze sculptures. Manufacturers will apply several coats of a barrier (paint-usually dark brown) to a bronze headstone plaque. Once the barrier film is dry, they will rub areas of the bronze plaque using a solvent to remove areas of paint to expose the top edge of the letters and sculpted design features to reveal the natural bronze coloring, which in turn must be coated with a clear barrier film. This provides a beautiful contrast to the dark brown background. One potential problem with photo etched bronze is the oxidation. When the piece oxidizes and starts to get darker the contrast produced by the rubbed patina will be lost and so will the image’s appearance.

PASSIVE FILM: The major characteristic of stainless is its ability to form a thin layer of protection called a "passive film" on its outside surface. This film results from a continual process of low-level oxidation, so oxygen from the atmosphere is needed for the passive film to exist. Once formed, it prevents further oxidation or corrosion from occurring. Even if chipped or scratched, a new passive film on stainless will form. It’s “self healing” like titanium. That’s why we use 316L stainless and titanium.

316 L Stainless Steel
Surgical Stainless

Surgical stainless steel is a variation of steel consisting of an alloy of chromium (12-20%), molybdenum (0.2-3%), and sometimes nickel (8-12%). The chromium gives the metal its sheen, scratch-resistance and corrosion resistance. The molybdenum gives corrosion-resistance, and helps maintaining a cutting edge.

Although there are myriads of variations in the recipes, there are two main varieties of stainless steel; martensitic and austenitic.

The austenitic stainless steels (300 series) were developed for use in both mild and severe corrosive conditions. Austenitic stainless steels are used at temperatures that range from cryogenic temperatures, where they exhibit high toughness, to elevated temperatures, where they exhibit good oxidation resistance. Because the austenitic materials are nonmagnetic, they are sometimes used in applications where magnetic materials are not acceptable.

Grade 316 has excellent corrosion resistance in a wide range of media. Better corrosion resistance than 302 and 304; resists sodium and calcium brines; hypochlorite solutions, phosphoric acid; and the sulfite liquors and sulfurous acids used in the paper pulp industry.

Etching Detail

Above is a photo of the corner of a Silver Memory™ tribute. The background (black) is a mirror polished surface. The silver letters are the result of etching into that mirrored surface.

Galvanic Corrosion Table

Most Corrodible

Least Corrodible

For harsh environments, such as outdoors, high humidity, and salt environments fall into this category. Typically there should be not more than 0.15 V difference in the "Anodic Index". For example; gold - silver would have a difference of 0.15V being acceptable.

For normal environments, such as storage in warehouses or non-temperature and humidity controlled environments. Typically there should not be more than 0.25 V difference in the "Anodic Index".

For controlled environments, such that are temperature and humidity controlled, 0.50 V can be tolerated. Caution should be maintained when deciding for this application as humidity and temperature do vary from regions.

Anodic Index	Metallurgy Index (V)
Gold, solid and plated, Gold-platinum alloy	0.00
Rhodium plated on silver-plated copper	0.05
Silver, solid or plated; monel metal. High nickel-copper alloys	0.15
Nickel, solid or plated, titanium an s alloys, Monel	0.30
Copper, solid or plated; low brasses or bronzes; silver solder; German silvery high copper-nickel alloys; nickel-chromium alloys	0.35
Brass and bronzes	0.40
High brasses and bronzes	0.45
18% chromium type corrosion-resistant steels (316L)	0.50
Chromium plated; tin plated; 12% chromium type corrosion-resistant steels	0.60
Tin-plate; tin-lead solder	0.65
Lead, solid or plated; high lead alloys	0.70
Aluminum, wrought alloys of the 2000 Series	0.75
Iron, wrought, gray or malleable, plain carbon and low alloy steels	0.85
Aluminum, wrought alloys other than 2000 Series aluminum, cast alloys of the silicon type	0.90
Aluminum, cast alloys other than silicon type, cadmium, plated and chromate	0.95
Hot-dip-zinc plate; galvanized steel	1.20
Zinc, wrought; zinc-base die-casting alloys; zinc plated	1.25
Magnesium & magnesium-base alloys, cast or wrought	1.75
Beryllium	1.85

You can see by the table above that the 316L stainless would be compatible with brass/bronze, even in a harsh environment. The same can be said for titanium and brass/bronze.