Writer: Ron Knight (Chemist)
Stain Less Steel
(is it really?)
What is stainless steel? Stainless steel is a family of iron-based alloys that must contain at least 10.5% chromium. The addition of chromium makes the material “passive” by creating a thin chromium oxide film on the surface of the steel that resists corrosion. This protective film is strong and chemically stable under aerobic conditions, which provide oxygen to the material surface, however if the stainless steel is exposed to an anaerobic process the passive film can be broken down and corrosion will occur.
There are a variety of stainless steel alloys that contain other elements including nickel or molybdenum. The reason for the variety of stainless alloys is that there are countless different applications and all forms of stainless steel should be considered. Stainless steels can be divided into the following three classes: austenitic, martensitic and ferritic.
Austenitic stainless steels have higher amounts of chromium and nickel than the other types. These steels offer a higher degree of corrosion resistance but they are not hardenable by heat treatment and have lower tensile strengths. Some of the more common austenitic series include: 302, 304, 310, 316 and 317.
Martensitic stainless steels can be hardened by heat treatment and therefore have a higher tensile strength than austenitic stainless steels but are not as resistant to corrosion. Some of the more common martensitic series is: 410, 416 and 17-4PH. The alloy 17-4PH has a variety of ingredients in it that gives this stainless steel the corrosion resistance of 304 and high strength.
Ferritic stainless steels have 12% to 18% chromium but have less that 0.2% carbon. These stainless steels are magnetic, non-hardenable by heat treatment and should not be used situations of high corrosion. A common ferritic series is 430.
For many applications, the problems of corrosion can be severe and the selection of the appropriate metal will make the difference of a long service life and an early (and sometimes catastrophic) failure. I am going to address one problem; galvanic corrosion.
Galvanic Corrosion: Corrosion is an electro-chemical action in which one metal is changed into another form. All metals have electrical potential and when two different metals are in contact with each other in the presence of an electrolyte, electrons will flow via the electrolyte from the more active metal to the more inert metal. This flow of electrons will continue as long as the active metal and electrolyte exist. The electrical potential is different for each metal so if two dis-similar metals are in contact with each other, electrons will flow and corrosion will exist. Please note Table I
304 Stainless (active)
316 Stainless (active)
Steel and Cast Iron
Hastelloy Alloy C
304 Stainless (passive)
316 Stainless (passive)
Hastelloy Alloy C (passive)
This table is easy to use.
The electrons will flow from the metals on the top of the list to the metals on the bottom and the farther apart the two metals are on the list the slower the rate of corrosion.
For example, Zinc and Steel will corrode faster than Zinc and Copper
If a passive 304 bolt is used to connect an aluminum plate with a brass plate both the aluminum and brass will corrode but the aluminum side in contact with the 304 will corrode faster than the brass and 304 side.
One of the first steps in correct material selection is to make an attempt to select fastener materials, which are compatible with the structure being joined . It can make a dramatic difference in the life of either the fastener and/or the base material if the incorrect fastener is selected.
A brief explanation of Table II
If an austenitic fastener (304) is used in a piece of zinc and/or galvanized steel the corrosion rate of the base metal (zinc or galvanized steel) will corrode at an accelerated rate as compared to using a “like metal” zinc and galvanized steel fastener. Notice what happens if you use an aluminum screw in anything but aluminum. The base metal does not corrode but the fastener will corrode at an accelerated rate. If you are trying to fasten an aluminum frame to something, you should use a zinc galvanized steel screw (or aluminum) screw because it will last longer than brass or stainless steel and the screw will be less expensive than the stainless steel screw. This selection can be confirmed by looking at table I. Zinc, aluminum and steel are located (on this chart) closer together than aluminum and 304 passive stainless.
Just using stainless steel does not guarantee to improve corrosion resistance. In addition to being expensive, stainless steel is less ductile than carbon steel, and does not have the tensile strength of carbon steel. Most stainless steels are not as hard as carbon steel and, therefore, will not be as resistant to abrasion.
Just like clothes, one size does not fit all. Look at the environment, application and service life desired and make good knowledgeable choices in materials selection.