Stainless steel provides superior corrosion and a surface area highly desirable for applications where the strength of iron and the resistance properties of chromium are needed along with a identifiable finish suitable for both industrial and commercial needs. So stainless steel is widely served for investment castings.
What Causes Corrosion?
Only metals such as gold and platinum are found naturally in a pure form – normal metals only exist in nature combined with other elements. Corrosion is therefore a natural phenomena, as nature seeks to combine together elements which man has produced in a pure form for his own use. Iron occurs naturally as iron ore. Pure iron is therefore unstable and wants to “rust”; that is, to combine with oxygen in the presence of water. Trains blown up in the Arabian desert in the First World War are still almost intact because of the dry rainless conditions. Iron ships sunk at very great depths rust at a very slow rate because of the low oxygen content of the sea water. The same ships wrecked on the beach, covered at high tide and exposed at low tide, would rust very rapidly. For most of the Iron Age, which began about 1000 BC, cast and wrought iron was used; iron with a high carbon content and various unrefined impurities. Steel did not begin to be produced in large quantities until the nine teenth century. Carbon steel can be defined as an alloy of a small content of carbon combined with well refined iron. Despite its various additions stainless steel still behaves as steel, it is not like the nickel alloys that are really alloys of a number of different metals, iron only being one. Even highly alloyed stainless steel grades such as 316 are a minimum of 62% iron.
Carbon steels without any protection will form a coating of rust which will in a sense protect the rest of the steel. So constantly removing the rust exposes a new fresh layer of steel to be attacked. This is called general corrosion. Various coatings of iron castings and steel castings will impede the rusting process, in particular painting, coating with zinc (galvanised steel), and epoxy resins.
Corrosion Resistance of Stainless Steel Casting
Corrosion is a natural part of exposure to the wind, water and environment. Pure elements such as iron inherently have a reactive nature with surrounding environments where alkaline or salt water is present. In fact, very few elements can be found naturally in their pure form. These sort of mixtures – alloys – have synergistic properties that retain the special characteristics of each of the elements and often become a new material with physical changes to its characteristics, even if both traditionally have the same defining features.
Rust is the process where iron molecules combine with oxygen in the presence of water. The result is a red flaky oxide that deteriorates easily, exposing more material to further corrosion. So iron casting and alloy & carbon steel are highly susceptible to this type of corrosion.
Stainless steel casting has the compositional ability to form a protective surface that prevents such corrosion. How does this happen? The chromium found in all stainless steel casting reacts quickly with oxygen environments, much the same as iron. The difference, however, is that only a very fine layer of chromium will oxidize (often only a few molecules in thickness). This incredibly thin barrier is highly durable and non-reactive and it adheres to stainless steel surfaces and won’t transfer or react further with other materials. It is also self-renewing in that it will replenish if
damaged or removed. Once oxidized, or passivized, stainless steel casting typically rusts at a rate of less than 0.002 inches in a given year.
And when kept in its best condition, stainless steel casting offers clean and bright surfaces ideal for many designs. The ultimate benefits of stainless steel casting include a long service life that will retain an attractive, clean finish.
Compared to other materials, stainless steel can be selected for investment casting for a number of different reasons, not just its resistance to corrosion. These include:
- Its aesthetic qualities: it can be polished to a satin or mirror finish.
- “Dry Corrosion” occurs to steel at higher temperatures where it oxidises or scales up. Stainless steel is far more resistant to this than ordinary carbon steel and grades such as 310 (25% chromium 20% nickel) were specifically developed for use at high temperatures.
- Non-contamination of the liquids stainless comes into contact with, because there is no coating to break down and dissolve.
- Weight savings: as thinner sections can be used, more innovative design structures can be used, with cost savings on foundations and platform weights.
- Many anti-corrosion coatings are fire hazards or the materials themselves have a low melting point.