Corrosion Prevention and Control in Mechanical Assemblies
Basics of design engineering call for preventing predictable failures during design.
| Published May 1, 2008
Most corrosion-control efforts center on coatings, but what if the design below that coating is flawed? That is the central focus of this article; there seems to be a general lack of awareness about design practices that will result in control of corrosion failures.
Aerospace and marine industries are probably the most familiar with the use of corrosion prevention techniques. The basic design guidelines include proper material selection, control of moisture, dealing with dead spaces, and identifying areas that can trap moisture. These design techniques should also be followed by other industries where corrosion may also be an issue.
The first mechanical design guideline for producing structures that are corrosion resistant is to understand the effect of galvanic potential on metal assemblies. Because this information has been around so long, some older charts are not up to date. One of the more current charts is published on the web by the Metal Boat Society1; an abbreviated version is shown in Table 1 at the right.
Table 1: The Galvanic Series
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About Carbon Fiber
Table 1 shows important details left out in some charts such as:
A.) Use of newer materials like carbon fiber.
B.) The galvanic potential that should be avoided is 0.2 volts (V) or greater.
C.) A potential difference exists between passive and active materials. An active surface includes a surface that is freshly machined. A passive surface has been protected most commonly by immersing it in an acid bath. This provides a protective oxidized coating for the metal surface.
Many designers do not realize that carbon fiber composites are very cathodic when used with metals. This could be because fiberglas and aramid fibers do not have this problem. Fasteners made from titanium, inconel, austenitic [carbon-iron component of steel] stainless steels and superalloys such as A286 may be used with carbon fiber. 2 Low alloy steels and martensitic [iron-carbon material] stainless steels are not acceptable. Wet fastener installation with a faying surface sealant — explained below — is required in all cases when using carbon fiber.
Another common error is the use of stainless steel fasteners in an aluminum assembly. From the chart the potential here could be as high as 1.0V. This can lead to very serious corrosion very quickly. The easiest solution in this case is to use a general-purpose thread sealant such as Loctite 511.3 Painting the threads with a sealant isolates the threads from the more reactive aluminum.
To Fay, a Shipbuilding Term for Join
The second guideline for preventing corrosion concerns the actual design configuration of mechanical joints and seams. All fixed mechanical joints and seams located in exterior or internal corrosive environments — including structures under fairings (an additional part of a structure added to smooth and reduce drag) — should be faying surfaces sealed with a suitable faying-surface compound. All removable panels and access doors should also be sealed with mechanical seals or by separable faying-surface sealing.4
To fay is a shipbuilding term meaning to fit closely, to join. A faying surface is a surface or faces of metal plates that fit so closely as to leave no space between. The danger here is that such a surface, when unprotected, is a site prone to corrosion. It is the classic blind pocket where moisture is trapped and corrosion begins. Composite construction should also avoid unprotected blind pockets, which may lead to moisture-induced blistering.
Suitable faying surface sealants are shown in Table 2 (below).
Table 2: Faying Surface Sealants
Notes: a) May be used uncured to enable disassembly of sealed joints. Source: NASA (5) |
A removable panel can also be protected from corrosion with a simple gasket assembly. Designs need to prevent over compression of the gasket with mechanical stops built into the design. Also, the gasket itself must be trapped in a pocket so assembly compression forces do not translate into adhesive shear failure in the bond between the gasket and the metal surface. The gasket and bolt are resilient members of the joint clamp load and must be engineered properly. Sharp edges are to be avoided.
Prevention and control of failure in mechanical assemblies during the design phase is much easier than after construction. It may possibly even avoid a product recall.
Fred C. Jensen is director of engineering at Patriot Engineering Co., which has been providing mechanical engineering solutions (design, FEA, machine/product development, machining-build) worldwide since 1979. You can send comments about this article to DE-Editors@deskeng.com.
References
1.) "The Galvanic Series", published in Metal Boat Quarterly, The Metal Boat Society, Sedro-Woolley, WA 98284; Summer 1997.
2.) "Composites," Vol. 1, Engineered Materials Handbook, ASM International; 1987.
3.) Loctite Worldwide Design Handbook, 1996/97, Loctite Corp., Rocky Hill, CT.
4.) "Space Station Requirements For Materials & Processes," NASA document, SSP 30233 Rev E 1995.
5.) "Process Specification for the Sealing of Joints and Faying Surfaces," National Aeronautics and Space Administration, Lyndon B. Johnson Space Center, Houston, TX; 2006.
Info:
Patriot Engineering Co.
Chagrin Falls, OH
patriotengineeringco.com