Ion microprobe investigation of decalibrated chromel versus alumel thermocouples: a quantitative SIMS analysis using indexed sensitivity factors and oxygen flooding

Abstract

Secondary Ion Mass Spectrometry (SIMS) was used to determine why sheated chromel versus alumel thermocouples decalibrate when exposed to temperatures above 1100°C. Two thermocouples, one sheathed in inconel-600 and the other in type 304 stainless steel, were studied. Quantified SIMS data showed that these thermocouples decalibrated because significant alterations in the elemental composition of the chromel and alumel thermoelements occurred. The extent of the observed alterations was different for each thermocouple and was influenced by the particular sheath material used in the thermocouple construction. Relative sensitivity factors, indexed by a matrix ion species ratio, were used to quantify SIMS data for three nickel-based alloys — chromel, alumel, and inconel-600 — and an iron-based alloy, type 304 stainless steel. Oxygen pressures ⩾ 2.66 × 10 -4 Pa in the sputtering region resulted in enhanced sensitivity and gave superior quantitative results compared to data obtained at instrument residual pressure. At sufficiently high oxygen surface coverage, the slope of the nonlinear sensitivity factor response curves for the nickel-based alloys approached zero. For the iron-based alloy, a set of linear sensitivity factor response lines was obtained with increasing oxygen surface coverage; both positive and negative slopes were observed, and the sensitivity factor values tended to converge on the unit value of the reference element (Fe) as oxygen surface coverage increased.