Tramp vs Trace: A Quick Lesson

Can a little extra element in your alloy make a difference? A BBC science article defines an alloy as a “mixture of two elements, one of which is a metal. Alloys often have properties that are different to the metals they contain. This makes them more useful than the pure metals alone. For example, alloys are often harder than the metal they contain.”

Brass is an alloy made of copper and zinc and is typically used in hinges and electrical and plumbing applications. Many statues are made of bronze, an alloy made of copper and another metal, usually tin. Copper-nickel alloys are used for marine applications because of their corrosion resistance. Over a year ago we discussed how manufacturers have to make sure they have the right alloy for the right application when it comes to ferrous metals. We talked about a new high fluidity zinc die casting alloy that was announced and covered by an ASTM Standard. In fact, ASTM International offers analytical chemistry standards, which are instrumental primarily in chemical analysis of various metals, alloys, and ores.

Do an internet search for a list of metal alloys and you will find almost every metal combination imaginable. The Aluminum Association offers a “quick read” list and infographic of aluminum alloys. It is amazing to me that sometimes just a small amount of one element can have an enormous impact on the resulting alloy. What’s the difference between Aluminum alloys 3015 and 3003 or 6063 and 6061? You better know if you care about the strength and integrity of your product.

Unfortunately, those small, maybe even minute, ‘extra’ elements – sometimes known as trace or tramp elements — may be contaminants and adversely affect your alloyed products.

Tramp or Trace – Which One Can Be a Problem?

The terms “tramp” and “trace”, however, are not always used interchangeably. Tramp describes an element that is not “specified” in the alloy grade, but an acceptable amount (trace) can be present without any detrimental effect on the alloy’s performance. For example: Stainless steel 304 can/will have up to 0.3% tramp copper, but above that amount, it can make the material more brittle.

Using the description “trace” element can mean a small amount of an element that appears in the alloy’s specification, and it is there for a specific reason. For example: A trace amount of zirconium (Zr) is specified in 7050 aluminum alloy, which helps to distinguish it from 7075 aluminum. The Zr is present (0.08 – 0.115%) in 7050 to reduce stress corrosion susceptibility.

Whether you are a producer of metals and alloys, or a fabricator using metals and alloys in your finished products, you should verify the exact alloy grade and percentage of alloying elements with x-ray fluorescence (XRF) technology. Handheld XRF analyzers can determine alloy composition and grade and help ensure material chemistry specifications are met. Today’s best practices include positive material identification (PMI) and testing 100% of critical materials as part of a quality assurance/quality control (QA/QC) program. Not knowing the exact composition of your materials – even just a trace of the unknown — can have catastrophic results.

Don’t trust that there are no unwanted tramp elements; just verify.