Rare-Earth Metal Long-Term Air Exposure Test

Rare-Earth Metal Long Term Air Exposure Test

Last updated photo: Nov. 2007

This test was conducted to demonstrate the effects of air exposure of the rare-earth metals over time. Started in January 2004, the test exposed samples of each of the 16 rare-earth metals (including scandium and yttrium) to normal household air. The metal pieces were cut and polished to provide a shiny surface for photographing. Over the test period, photos were taken of the sample set and are displayed below.

Each thumbnail photo can be clicked to show a larger image.

Start of study - Jan. 31, 2004	2 hours later...Europium starts to tarnish	12 hours later...	2nd day: Lanthanum starts to turn dark	Day 3: Praseodymium starts to turn color
Day 4: Europium shows yellow corrosion	Day 5: Cerium becomes dark	Day 6	Day 7: Europium corrosion accelerates	Day 8
Day 9	Day 10: Europium corrosion starts to stack up	Day 12: Lanthanum shows signs of spalling oxidation	Day 16: Europium is covered with corrosion	Day 17
Day 18	Day 20	Day 22: Europium appears to be completely corroded	Day 25	Day 32: Neodymium starts to turn dark
Day 36: Europium corrosion removed from test plate	Day 50: White lanthanum oxide is clearly visible	Day 71: Cerium and praseodymium show spalling	Day 92	Day 113: Thulium shows noticeable loss of shine
Day 132: Lanthanum corrosion accelerates	Day 191: Praseodymium and neodymium show oxide colors	Day 335: Lanthanum completely corroded	Day 391: Samarium starts to tarnish	Day 586 (19 months): Cerium, Praseodymium and Neodymium are almost gone, but most others are still shiny
3 Years Later: Samarium has lost most of its shine, but the others have not changed in over a year. The corroded metals have been removed. Terbium and Lutetium samples were lost during handling.	4 Years Later: Samarium is no longer shiny but the bulk of the metal is still intact. No changes observed on any other metal samples.

Why is this test being done? It is known that some of these metals will corrode in air especially when moisture is present. Europium, lanthanum, cerium, praseodymium and neodymium are known to corrode fairly quickly and as a result are usually stored under argon or mineral oil to protect them. The uncertainty was how long would it take, and would the other metals corrode also.

This test showed that these metals started to corrode in roughly the following order:

Europium (completely corroded after 22 days)
Lanthanum (completely corroded after about 11 months)
Cerium (completely corroded after about 1 year)
Praseodymium (completely corroded after about 1 year)
Neodymium (completely corroded after about 1 year)
Thulium
Samarium

Some observations:

When the europium and lanthanum started to corrode, they did so relatively slowly then after a while seemed to accelerate. For the reactive metals (europium, lanthanum, praseodymium, cerium, neodymium) the oxide "spalls" off which constantly exposes fresh metal, allowing the corrosion to continue unabated.

Europium lost its shine after about 2 hours.

Lanthanum turned blue within a few hours of the start of the test before turning a dark gray color.

Since the test was started in the winter season the air was probably quite dry for the first few months then became humid during the summer months. The dryness may have delayed the corrosion rate for the first few months.

The other metals not mentioned yet (scandium, yttrium, gadolinium, terbium, dysprosium, holmium, erbium, ytterbium, lutetium) appear as shiny after 1 year as they were at the start of the test.

For the Day 391 photo the europium corrosion product was placed back on the test plate. After this time the corrosion product is now off-white in color. It could possibly have absorbed carbon dioxide to form the carbonate.

Day 586: After over a year and a half, the stable rare-earths are still shiny, as shown in the Day 586 photo. The cerium oxide seems to bind together in small plates, unlike the other oxides which form powders.

Three Years: After close to three years, most of the metals are still shiny. The reactive metals are completely gone and their reaction products were removed from the plate for this photo. Unfortunately for the experiment the Lutetium and Terbium samples were lost during handling for the photo, but did still have their shine when last observed. Samarium and thulium have shown very slow but clear signs of corrosion.

Four Years: After close to four years, the only noticeable difference was that samarium became a bit darker. The other metals are either still shiny or in the same state they were the previous year.

Test conducted by David Hamric, Metallium, Inc.