The Right Material Matters: Ensure Performance and Reliability with Expertly Forged Parts
Corrosion of a metal is a chemical or electrochemical process where the surface atoms of a solid metal react with a substance that is in contact with the exposed surface. The corroding medium is usually a liquid, but can be a gas or a solid. All structural metals corrode to some extent in natural environments. Bronze, brass, most stainless steels, zinc, and pure aluminum corrode so slowly that long service life is normal, without protective coatings. Corrosion of structural grades of steel, the 400 stainless series, and that of some aluminum alloys is likely to proceed rapidly unless the metal is protected against corrosion. Corrosion of iron and steel is particularly pertinent since billions of dollars are lost annually due to steel corrosion.
Electrochemical corrosion in metals in a natural environment, be it atmosphere, water, or underground, is caused by a flow of electricity from one metal to another, or from one part of a metal surface to another part of the same surface, where conditions permit the flow of electricity. For this flow to take place, either a moist conductor or an electrolyte must be present. Simply put, an electrolyte is a substance that can conduct an electric current when melted or dissolved in water, and its presence is necessary for corrosion to occur.
If oxygen and water are both present, corrosion will normally occur on iron and steel. The rate of corrosion will be sped up by the velocity or acidity of the water, a temperature increase or aeration, or the presence of certain bacteria. High alkalinity will retard the rate of corrosion. Water and oxygen are the determining factors, however.
In an acid environment, even without oxygen, the metal will be attacked by corrosion.
Galvanic corrosion involves an electrochemical corrosion reaction between dissimilar metals, according to the galvanic series that lists a number of common metals and alloys and their tendency to corrode when in galvanic contact. The greater the gap between two metals or alloys, the more likely corrosion of the “weaker” one. For example, commercial quality aluminum will corrode before carbon steel, which will in turn corrode before 400 series stainless, which will corrode before type 304 stainless, then type 316 stainless. There are many cases where types 316, type 317, are just not satisfactory due to corrosion in a specific environment, hence we need to move to highly alloyed nickel alloys.
Pitting, although not necessarily affecting a large area, can render a part unserviceable. It may occur where large surface areas are covered by mill scale or deposits of various kinds. Small pits may be exaggerated by fabrication, or a local lack of oxygen.
Water and oxygen and chlorides/fluorides, together with stress on the metal or alloy, may do irreparable damage. Coatings and corrosion inhibitors have a positive part to play, when thought through at the design stage, in protecting the metal or alloy. There are myriad combinations of protection, as there are myriad combinations of corrosive environments and alloys designed to resist them.
In the chemical processing industries, where harsh chemicals, high temperatures, and high pressures are encountered, structural metallic parts, pipes, pumps, and valves must survive long-term exposure to all conditions, from mildly to severely corrosive. And when we think of corrosion, we tend to picture stainless steel doing the job. But many media are way too much for stainless steels, even the type 317 with around 3.5% molybdenum. So, in such cases, we need to move up to the high nickel alloys, with molybdenum, copper, chromium, columbium, titanium, and aluminum additions for age hardening purposes.
Chromium and molybdenum help with pitting, while nickel at over 25% in reducing environments resists stress corrosion cracking, and copper helps with resistance to sulfuric acid. Stronger alloys and heat-treated alloys, are generally more susceptible to stress corrosion cracking. Fabrication of certain alloys may adversely affect their corrosion resistance.
It is evident that corrosion has been instrumental in the development of a very broad range of alloys to combat the phenomenon. Much information is available on the alloys that have been developed and are still being developed. Along with the alloys come the experts, the corrosion engineers, whose input into the selection of an alloy for a job will prove invaluable. This is not an easy science, but knowledge of the causes and effects of corrosion, together with the choice and production of the alloys to combat it, will serve to reduce its consequences. At AMFG we are familiar with the causes and consequences of metallic corrosion. And with the production and processing of the materials that have been developed to combat corrosion. If we don’t know the answer, we can more than likely find it.
