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Application Guide catalog - 7347 Application Guide

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The Art of TemperatureSensing

mocouple inspections before completethermocouple failure occurs.One or the other of the following condi-tions is usually found on examinationof Chromel-Alumel

Metallurgical Factors

® thermocouplesexperiencing early failure. 1. Selective Oxidation — sometimesreferred to as “green rot”—is typified by a greenish surface or subsurface scale that develops in nickel-chromium alloys, when subjected to a marginallyoxidizing environment at hightemperature.The formation of a thin nickel film willgive the non-magnetic KP magnetic properties.In applications where an abundantsupply of oxygen is available, or wherethere is none at all, green rot does notoccur. Reducing atmospheres suchas pure dry hydrogen, will not adverse-ly affect nickel thermocouple alloys.When green rot is apparent, check forthe following possibilities:•Protecting tubes having too smallan inside diameter, or too great a length-diameter ratio, which prohibit a reasonably free air circulation between the insulated (ceramic) thermocouple element and the tube wall.•Leaks in protecting tubes withrefractory metal (titanium, colum-bium, tantalum) wires inside andsealed at the “cold end.” The pur-pose of the refractory metal wire is to “sop up” all available oxygen creating an oxygen free atmosphere. The atmosphere will become partiallyoxidizing when the refractory metal wire cannot absorb all the oxygensetting the stage for green rot.•Improperly degreased protectingtube interior. Residues of greases and oils will decompose and releasesulfur, a deadly enemy of nickel alloys.•Leaky protecting tubes in reducing(hydrogen for example) or carbona-ceous gas containing furnaces, where furnace atmospheres canpenetrate the tube and will causepartially oxidizing conditions withinthe leaky tube.•Presence of zinc (galvanized tubes)or zinc-containing alloys, such as brass. Vapors from these metalswill accelerate deterioration ofnickel thermocouple alloys.
Inter-granular attack due to selective oxidation in8-gauge wire. (Unetched, X100) Premature thermocouple failure is mostfrequently a result of contamination orcorrosion due to uncontrolled furnace atmosphere, unclean or leaking protec-tion tubes, or some other factor relatedto improper installation or operation. Damaging conditions can usually bedetected and corrected through a pro-gram of frequent and systematic ther- 2. Sulfur Attack — Sulfur is particularlyharmful to high nickel alloys including KN. In heat-treating operations, sulfurmay come from various sources suchas furnace atmospheres, oil, mortar, cements and asbestos. In Type Kthermocouples, sulfur attack oftenreveals itself in breakage of the KN wire. Thus, when normally ductile KN wire appears to have become brittle in service, that is, if surface cracks appear when it is bent with the fin- gers, it is likely that sulfur corrosion has occurred. In case of doubt, the presence of sulfur can be determined positively by performing any one of several chemical tests.A simple test for sulfur in a suspectedmaterial is to immerse a sample of the material in a solution of 20 percent hydrochloric acid containing a fewpieces of metallic zinc. If sulfur is present in the sample, it can beidentified by the characteristichydrogen sulfide odor of rotten eggsthat will evolve. Also, moistened leadacetate paper held over the top of the test solution will turn brown or black ifsulfur is present in the sample.Where there is evidence of sulfurattack in the thermocouple, it shouldbe replaced, and an attempt made to eliminate the source of sulfur. If elim-ination of the source is not feasible, then the thermocouple should be com-pletely isolated from the contaminat-ing material. The possibility of a leakin an existing protection tube should not be overlooked.
Transverse inter-granular cracks due to sulfurattack in 8-gauge wire. (Electrolytic etch, X35) Alumel ® is a registered trademark of Hoskins Manufacturing Company. 44