The proper design to minimize corrosion is as important as the selection of materials. In many structures, a lack of design consideration given to corrosion control is the weakest link in corrosion control. Designers should have skills to determine the mechanical properties and strength required to ensure that a structure is resistant to corrosion. A background and solid understanding of the corrosion process is important as well. While selecting materials, the designer has to consider the particular environment, use, and the corrosion control parameters. These include:
Designing structures and parts to prevent or control corrosion is more cost-effective than waiting for the equipment to fail in service. Close communication between designers and corrosion engineers can be very beneficial and should be ensured in applications where corrosion is likely to be an issue. (For more about the design process, read Corrosion Control Considerations in the Equipment Design Process.) There are a few key design rules that can be followed to help prevent corrosion:
Adjust Wall Thickness
Because corrosion involves the degradation of materials, the process continuously eats up the material and decreases its thickness. Therefore, part of corrosion-resistant design involves making allowances for this reduction in the thickness in pipes, storage tanks and other parts. A general method is to make the wall thickness twice of that which is required for the desired life of the structure. However, the wall thickness must meet mechanical requirements for stress, pressure and weight.
This general rule of using doubled wall thickness adds extra cost and weight. Therefore, detailed financial comparisons should be made to choose among these options. This rule does not need to be followed if there is reliable corrosion data and effective monitoring systems. For example, we might use different corrosion allowances for the upper and lower regions of a tall vertical vessel.
Ensure that there's Adequate Drainage
Tanks and other storage containers should be designed in such a way that they can be easily drained and cleaned. Therefore, all transitions should be smooth, and taps should be located so that the tank can be completely drained.
Minimize Bi-Metallic Corrosion Cells
Avoid galvanic corrosion by using similar metal throughout the structure, if possible, or by avoiding electrical contact by insulating different materials. Those components that are more prone to corrosion should be easily replaceable. Special parts (wasters) can also be used to attract the corrosion, thus protecting other parts. To avoid crevice corrosion, seals should be used and pressure should be adjusted on the gaskets to prevent liquid penetration inside the crevices. In this way, it is possible to avoid the presence of stagnant water in the crevices and narrow gaps. (For background information, see An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion.)
Avoid Differential Aeration Cells
Differential aeration should be avoided. For the components immersed in water, sufficient aeration should be ensured to cause passivation, which slows the corrosion. Otherwise, aeration should be prevented as much as possible. Similarly, for the structures that are exposed to the atmosphere, easy drainage and an ample supply of air should be ensured, and vice versa for the porous surfaces or structure having cavities—they should be properly sealed.
Use joints that suppress corrosion problems as well. For example, butt welds should be used instead of overlap joints. Welding should be preferred over riveting in tanks and containers, but welding should be avoided if the materials are protected by paint or zinc coating. It is good practice to post-treat the welds as well. They may be blast cleaned and coated with tape, paint or thermal spray.
Minimize Temperature Gradients
The equipment for heat transport should be designed so that surface temperature varies as little as possible. Cold and hot spots should be avoided. Superheated spots are prone to thermogalvanic corrosion and cold spots can enhance local condensation, which leads to corrosion. Therefore, the thermal gradient should be kept to a minimum.
Minimize Stress Gradients
Stress concentrations in the components exposed to corrosive mediums should be avoided, especially when using materials susceptible to stress-corrosion cracking. Therefore, designers should aim for simple geometry, as abrupt changes in dimensions can provide sites for stress concentration.
The surroundings should be considered to minimize the consequences of corrosion. This involves making sure that separate systems do not impair the environments of others. For example, if a copper alloy corrodes and the moisture containing copper ions come in contact with an aluminum component, it will result in galvanic corrosion. Polluting plants or the environment should be downwind to the structure to be protected if possible, so that air doesn’t contain harmful impurities.
Minimize Turbulence in Pipe Systems
In piping systems, the designing should be such that the flow has minimum turbulence. Turbulent flow enhances corrosion, so the flow should be laminar and the thickness of the structure should be great enough that it can bear the effects. The number of bends should be as minimal as possible and it is essential to round the sharp bends. Sharp bends should be avoided in piping systems with high velocity fluids or solids in suspension to prevent erosion corrosion. (Discover more tips in Combating Cavitative Corrosion and Erosive Corrosion.)
The Key: Avoid Heterogeneity
The most general rule for proper design is to avoid heterogeneity. Heterogeneity consists of different metals, uneven stress and temperature distribution. Sharp corners should be avoided because they are difficult to paint with uniform thickness. Complex geometries and narrow gaps impede surface treatments like painting, thermal spraying and blast cleaning; increase the cost; and make it difficult for cleaning and drying. All the relevant codes and standards should be met. Rules for minimum gaps between profiles are given in ISO 12944-3:99.
Procedures for testing and storage of parts and equipment, operating, and maintenance should be specified. (Described in detail in the article Temporary Corrosion Protection during Storage, Transportation and Handling.) Hence we can see that proper design to control corrosion is as significant as the selection of materials. We should aim for simple design, and avoid heterogeneity as much as feasible.