by Norman D. Groves The Carpenter Steel Co.
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Design for Corrosion Resistance, Not Attack I HERE are eight major classifications of corrosive attack: general, intergranular, concentration cell, galvanic, pitting, stress corrosion cracking, corrosion erosion, and dezincification. Some authorities break these down a little finer and add fretting and crevice corrosion to get ten types. But regardless of the exact number of types, each one can ruin a piece of equipment that represents a lot of time and effort on the part of a designer. Often, the fault lies in the material selected for the environment, but, just as often, it is the fault of the design itself. Designers make things tough for themselves by designing a piece of equipment in a manner that promotes corrosive attack rather than minimizes it. Many engineers are so involved with the mechanical aspects of the design that they give only little consideration to the end corrosive conditions. When the equipment is all designed, the plan is turned over to a materials engineer to select materials that will withstand the corrosive attack. And if the part fails, the materials engineer is blamed for not selecting the proper materials. This would be true only if the failure was due to general corrosive attack. In several types of corrosive attack, configuration plays a major role in resistance. In others—intergranular and galvanic—method of assembly and insulation, respectively, are the controlling factors. In fretting corrosion, relative motion, bearing pressure, and lubrication are major considerations. Few designers would disclaim responsibility for shape, method of assembly, relative motion, bearing pressures, and lubrication. Because these are the factors that directly influence corrosion rates in the majority of types of corrosion, corrosion resistance is a design problem. Circle No. 28 on Readers' Service Card
Basic Design Practices Here are some basic practices that can be followed to improve resistance to all types of corrosive attack. Make Cleaning Easy. Probably the one most important thing a designer can do to promote long equipment life is to make cleaning easy. It is generally believed that the corrosion-resisting properties of stainless steels are due to a very thin film of oxide that completely covers the surface and prevents further oxidation. Attack occurs when this film is broken or contaminated by dirt or scale adhering to the surface. A freshly cleaned, machined, polished, or pickled article will acquire this film quickly. Frequent cleaning keeps scale and dirt from accumulating and allows re-formation of the protective coating. Sharp corners are never easy to clean; round them or provide for a fillet (Figure 1). Provide ample radii on all pipe bends and vats. Avoid built-in dirt catchers. Specify a finish consistent with the accessibility of the equipment for cleaning. For example, if you are designing food-processing equipment with pipe interiors that can be cleaned only by flushing, require a more highly polished surface than an exposed readily cleaned surface subjected to the same fluid. Dirt doesn't stick to smooth surfaces as readily as to rough surfaces. Design for Replacement. This is the old maxim—If you can't beat 'em, join 'em. Ask yourself, "Is it actually necessary for this part to last forever in this corrosive atmosphere?" It may cost much less in the long run to design the part for easy replacement when it does succumb to chemical attack. For example, the heat exchanger section of a vessel such as a fractionating tower should be replaceable. The I/EC
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metal may be sufficiently corrosionresistant for the rest of the process, but the elevated temperatures of the heat exchanger will increase the corrosive attack. Any section of a system that may be subjected to higher corrosion rates than is normal throughout the rest of the system should be replaceable. Let the Fluid Work for You. A good example of this is illustrated in Figure 2. When corrosive fluid is added to a vat in a more highly concentrated form than is present in the vat, introduce the concentrate into the center of the vat, as far away from the metal sides as possible. The fluid will then dilute the concentrate before it comes in contact with the sides of the vat. Use Streamlined Design. Streamlined design can prevent corrosive attack from many angles. It makes equipment easy to clean. Smooth fluid flow does not break down the protective oxide film as does the mechanical action of induced turbulence. Avoid abrupt constrictions in fluid lines. There are all sorts of constrictions to look for: weld joints with backup rings, undersize valves, butt joints, etc. Sediment can collect in the lee of constrictions. They form natural cracks and crevices and the edge of the material is exposed to the corrosion. In Case of Failure
Unfortunately, there is no cure-all that can be applied if, after following good basic design practices, the part still fails in service. The most important part of the correction procedure is to diagnose the cause. Too often, designers are expected to correct the cause without knowing the disease. An example or two should illustrate this. An aircraft hydraulic pump manufacturer had trouble with the standORKBOOK FEATURES
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Figure 1 . Sketches showing good and b a d configuration including sharp corners, projection, pockets, etc.
a r d A N externally splined coupling, which was exhibiting excess wear, resulting in a higher t h a n normal replacement rate. T o alleviate wear in the coupling, the designer increased the hardness of the spline teeth. This only transferred the wear from the coupling to the internal splined m a t i n g p a r t — a m u c h m o r e expensive p a r t to replace. After considerable trouble, the p r o b lem was finally solved by redesigning the coupling for lubrication that reduced the fretting corrosion. Another example, a 35-mesh wire screen basket is used to process uran i u m ore. T h e baskets failed u n d e r conditions that on the face a p p e a r e d to be crevice corrosion. At the j u n c t i o n of the frame m e m b e r s a n d the wire mesh, the mesh was failing. A thorough investigation proved the sharp edges of frame m e m b e r s were notching the wire a n d causing fatigue failure—-not corrosive failure at all. By simply r o u n d i n g the corner on the frame m e m b e r s , the problem was solved. I n case of failure—diagnose carefully. Be sure w h a t it is you are trying to correct. T h e defense against corrosive attack is not necessarily a material with more corrosion resistance, but, rather, the selection of a combination of configuration a n d material and fabrication techniques that is more resistant to the source of trouble. W h e n You K n o w the Cause
W h e n the cause for a failure has been determined, some cures lie within the realm of the e q u i p m e n t designer and some d o not. Some corrective measures m a y sound like duplication, because some causes promote several types of failure. Poor welding, for instance, m a y 76 A
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provide crevices for crevice corrosion a n d constrictions for the build-up of sediment a n d p r o m o t e intergranular corrosion. Intergranular Corrosion. T h e r e is not m u c h a designer can do for intergranular attack, as this is a m a t t e r of material selection or p r o p e r heat treatment. T h e only correction factor he can apply is the use of some other m e t h o d of fastening. I n t e r g r a n u l a r corrosion d u e to welding often can be overcome by specifying the proper grade, such as a low c a r b o n or stabilized grade of the stainless steel. Concentration Cell Corrosion. This can occur wherever an ion concentration or a t e m p e r a t u r e differential exists—such as stratified layers of fluids—or stagnation or eddying occurs. O n e of the most c o m m o n instances of concentration cell corrosion is called crevice corrosion. Often, crevice corrosion is not properly identified because the area of attack does not look like the p o p u l a r concept of a crevice. A crevice has no particular dimension, but r a t h e r it is wherever the flow of the corrodent is impaired. T o have crevice corrosion, the corrodent must become concentrated . A thermal gradient or normal diffusion will cause this concentration. Incidentally, no mechanical joint, such as a t h r e a d e d or rolled connection, is tight e n o u g h not to be a crevice. Sediment accumulation can create a crevice. O n e of the c o m m o n areas of crevice corrosion is at welded pipe joints. Properly designed a n d fabricated pipe joints can alleviate the problem. Avoid b a c k u p rings t h a t can cause crevices. T h e r e are several good designs a n d materials available to reduce the h a z a r d of crevices in welded joints. Fretting Corrosion. This is strictly the designer's problem. I t is caused by oscillatory motion between load-bearing parts. Elevated bearing pressures, such as a ball bearing riding on a flat track, increase fretting corrosion. T o alleviate fretting corrosion, the following steps can be taken : 1. Distribute the load over a larger area. In a bearing this can be accomplished by changing the radius of contart
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 2. Introduce concentrated liquid into center of v a t , so liquid can dilute b e f o r e reaching edges. A n other important factor to prevent g a l vanic attack is the insulation of the v a t from the piping
2. Increase the strength of the material, as fretting is more prominent in the softer material. Caution should be taken not to transfer the fretting wear to the other part. 3. Increase the pressure between the parts to decrease slippage. 4. Roughen the mating surfaces to avoid slippage. 5. Provide lubrication to put a film between the mating surfaces. Erosion Corrosion. Design for l a m i n a r flow. Avoid turbulence, if at all possible, as the mechanical action tends to break down the protective film. This boils down to a straight do a n d d o n ' t proposition. Don't use sharp bends; do use gradual radius bends. Do use oversized valves, so no orifice or Venturi effects are created. Don't use butt joints; use tube reducers for both expanding and contracting flow lines. Don't use backup rings that constrict pipes. Use long radius bends and elbows for liquids containing solids in suspension. Above all, design for laminar flow.
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