Does choosing the best material for the job have you stymied?

It isn’t an exact science and it is often necessary to be guided by past experience and to make informed judgments based on sometimes incomplete information or data. Consider using the expertise of material consultants, or the material supplier’s own technical staff. But for all those DIY’rs out there, here are some thoughts on how to approach this often complex decision.

Get in the ball park

The easiest starting point is to consider the most likely type of general corrosion. There is much data available on general corrosion resistance and rates. These sources include publications by organizations such as NACE International (www.nace.org), MTI (www.mti-global.org) and the many websites of material suppliers and producing mills. Our own website (www.RolledAlloys.com) offers a considerable amount of data on both aqueous and high temperature corrosion. Unfortunately, there is no real universal method or conditions for reporting corrosion rates so it is important when making comparisons that you understand the test conditions and sample parameters. For most applications, a general corrosion rate below 20 mpy (0.020”/year) is considered acceptable.

The next consideration is the potential for localized corrosion. Localized corrosion includes pitting attack, crevice corrosion, galvanic effects, and concentration cells. While general corrosion is uniform and predictable, localized corrosion is very difficult to quantify. In addition, once it begins it can propagate rapidly, so it is desirable to select a material or design that is not susceptible to this form of attack.

Once a group of alloys is identified that offer suitable corrosion resistance to the environment, other factors need to be considered. Product form availability is often the next parameter to consider. Some materials are only available in certain forms or dimensions. This could be because of metallurgical or commercial reasons. Making a component or system of components may require the use of multiple alloys, but be careful not to create combinations that could result in significant galvanic effects.

Fabrication issues should be considered next. Some alloys do require more time for machining or welding than others. While these considerations won’t normally be reason to rule out a material, ease of fabrication can be an overall cost factor. Strength is another parameter to consider that typically won’t rule a particular material in or out. However, higher strength materials might afford the opportunity to reduce component weight, raw material costs (less material), or reduce heating or cooling costs.

At Rolled Alloys, we sometimes say that Material Availability is THE most important property. If you can’t get the material you want when you need it, it isn’t of any use. On initial projects, longer lead times may not be of real concern. But for maintenance and repairs, designing using commonly inventoried items can help lower operating costs and down time.

Of course there is a cost…

Cost should actually be the last consideration in the material selection process. The cost determined after the ideal material for the job has been decided. But in reality, we don’t choose the ideal material simply as the one that will last the longest or do the best job. Our goal should be to optimize the Design Life Cycle Cost. Any benefit from using a higher cost material must be offset by increased service life, reduced maintenance costs, superior product quality, less down time and less risk to the environment.

Material Selection

Often, these hidden costs are overlooked in favor of a lower initial capital cost. But in the longer run, material selection can have a much greater impact on operating costs than we at first imagine.

 

-Paul Whitcraft