Designing With Metals: Stainless Steel and Corrosion Resistance

The textbook, page 459, describes stainless steel as a steel alloy containing chromium and nickel, and being highly resistant to corrosion. Stainless steel is commonly used in architectural applications such as metal roofing, wall panels, railings, sanitary surfaces, flashing, shelf angles, lintels, masonry veneer anchors, and hardware and fasteners exposed to moisture. Stainless steel is 100 percent recyclable, and stainless steel made today typically contains 65 to 80 percent recycled content.

What is Stainless Steel?
Steel containing approximately 10 percent or more chromium qualifies as stainless. The chromium at the surface of the metal combines with oxygen from the atmosphere to form a thin, clear oxide film. Once formed, this oxide layer itself does not react with most corrosive elements and in effect forms a tight, protective seal around the metal. Under normal conditions, even if the oxide layer is scratched or damaged, the oxide layer will re-form, essentially healing itself and maintaining protection of the underlying metal. In the technical literature, this oxide layer is said to create a passive barrier on the surface of the steel.

Shown from front to back are flashing samples made from stainless steel, copper, and galvanized steel. Compared to copper, stainless steel is harder and stiffer, in these respects making it more difficult to work with as a flashing material. Copper has a more distinctive color and produces runoff that may cause staining. Galvanized steel is the least expensive of the three, but also is not as long lasting. (Hues in this digital image have been slightly intensified to accentuate the differences in color between the three metals.)

What Are the Common Types of Stainless Steel?
Different stainless steel alloys are distinguished by the varying amounts of chromium and other metals added to the steel. Within the range of common stainless steel alloys, the higher the percentages of chromium and nickel, generally speaking the more corrosion-resistant the alloy. In architectural applications, the most commonly specified stainless steel alloy is Type 304. Type 304 stainless steel has 18 to 20 percent chromium, 8 to 12 percent nickel, and smaller amounts of other elements. Chromium provides the base level of corrosion resistance. Nickel adds additional corrosion resistance and improves the ductility of the metal. (Steel alloyed with just chromium tends to be hard and brittle.)

In highly corrosive environments Type 316 stainless steel is recommended. This alloy differs from Type 304 in the addition of 2 to 3 percent molybdenum, and an increase in the percentage of nickel. The result is greater resistance to chlorides and other corrosives. This alloy is particularly noted for its resistance to a form of corrosion called pitting that is common in marine environments. Type 316 stainless steel is also more expensive than Type 304.

Where stainless steel requires heavy welding, variations on these two alloys may be used. At the high temperatures of welding, carbon in the steel combines chemically with the chromium, rendering the chromium unavailable for the formation of the protective oxide coating. Without the ability to form this coating, corrosion-prone areas form in the areas surrounding the weld. In Type 304L and Type 316L stainless steel, the carbon content is reduced from a maximum of 0.08 percent to less than 0.03 percent. The reduction in carbon ensures that chromium remains available for formation of the oxide coating. As a side effect, the yield strength of these alloys is also reduced, from approximately 30,000 psi (205 MPa) to 25,000 psi (170 MPa).

The most common stainless steels are also sometimes referred to as austenitic. This term describes the crystalline metal structure of these alloys (face-centered cubic). The austenitic stainless steels include the 300 series alloys as well as some less common 200 series, lower-nickel, alloys. In comparison to other alloys, austenitic stainless steels are higher in chromium and nickel, low in carbon, and highly corrosion resistant. They have rates of thermal expansion 30 to 50 percent greater than normal carbon steels. They are also distinguished by being non-magnetic, although some can exhibit mild magnetic properties after cold working.

One limitation of austenitic stainless steels is that they cannot be hardened by heat treatment. Type 410 stainless steel is a martensitic alloy containing roughly 12 to 14 percent chromium, little or no nickel, and up to 0.15 percent carbon. Type 410 stainless steel may be hardened, though it also has less corrosion resistance and is less ductile in comparison with the 300 series high nickel alloys. Case hardened Type 410 stainless steel may be used, for example, in the manufacture of self-drilling or self-tapping screws for fastening to steel or concrete, where Type 300 series alloys would lack sufficient hardness to cut through these dense materials.

Some architectural stainless steel may also be referred to as 18-8 stainless steel. This term refers to Type 304 and a few other closely related alloys, all of which have approximately 18 percent chromium and 8 percent nickel, and all of which share similar levels of corrosion resistance and other physical properties. In many cases the term 18-8 is used interchangeably with Type 304.

Two stainless steel fasteners from a manufacturer's catalog listing are shown. Note that the self-drilling screw for fastening into wood is made from 18-8 (Type 304) stainless steel, while the fastener designed to cut threads in much harder steel is made from hardened Type 410 stainless steel.

What Is the Difference Between "Active" and "Passive" Stainless Steel on the Galvanic Series?
The galvanic series is a list of metals arranged in order of their electrical potential. A simple version of the galvanic series is shown in Figure 16.55, page 604, of the text. When two metals are in contact and in the presence of moisture, their relative locations within the series indicate the risk of corrosion due to the flow of electric current between them. The closer the two metals on the list, the less the difference in electrical potential, and the less the risk of corrosion; the further apart the two materials on the list, the greater the risk of corrosion.

Many galvanic series lists show stainless steel in two locations, one for passive stainless steel and another for active. These terms refer to the presence or absence of the protective oxide coating that normally forms on the surface of the stainless steel, as discussed above. Under any normal circumstance, stainless steel used in architectural applications will exhibit this oxide coating and thus its galvanic properties should be referenced from its passive location within the galvanic series. While it is possible for the surface of stainless steel to become active under certain conditions, such circumstances are not common to architectural applications, and references to active stainless steel on the galvanic series should normally be ignored.

This is the first article in an occasional series on desining with architectural metals.
Next: Dissimilar Metals And The Galvanic Series

More information:
The textbook discusses architectural uses of metals on pages 458 - 460.
Why Is Stainless Steel Stainless? provides an easy to understand explanation of stainless steel basics, and links to additional informative sites.
Corrosion, Stainless Steel is a clear and technically detailed account of corrosion mechanisms in stainless steel.
Prevention of galvanic corrosion by design provides a brief summary of design strategies for avoiding galvanic corrosion between dissimilar metals, and lists links to related information.
Stainless Steel Information Center offers extensive reference information on the properties and uses of stainless steel.
Architectural Metals, by L. William Zahner (John Wiley & Sons, Inc., 1995), provides in-depth technical and design information on stainless steel and other architectural metals.