Details of Pure Nickel Wire in Industrial Applications

Pure nickel wire is drawn from commercially pure nickel, and the phrase describes a family and not a single product. The two grades that supply almost all of it differ only in carbon, they are covered by different UNS numbers, and the choice between them is made by the process the wire goes through and not by the duty it ends up in. Everything else about the wire follows from two of its own properties, and those two properties decide which of the familiar applications it can actually carry.

The applications are usually listed as electrical components, heating elements and batteries. That list is accurate as far as it goes, and two of the three come with a published qualification that a buyer needs before the wire is ordered, because the qualification is what decides whether the application works at all.

Commercially pure nickel is described in its bulletin as 99.6 % pure, and the specification it is sold to sets a minimum of 99.0 % for nickel plus cobalt. The commercial figure and the specification figure are two different statements, and the element that separates the two grades in the table below is carbon, at 0.15 % maximum against 0.02 % maximum. That single difference decides whether the wire survives a bright anneal without the risk of graphite forming in it.

The order line should carry the UNS number and not only the purity figure, because a certificate showing 99.0 % minimum and a certificate showing a commercial 99.6 % are both correct for two different grades. Where a wire will be bright annealed after drawing, the low carbon grade is the one to specify, and the reason is the graphitization band and not the electrolysis duty: prolonged exposure between 800 °F and 1,200 °F, which is 425 °C to 650 °C, precipitates graphite and can severely compromise the properties.

Nickel wire is chosen for what it resists, and its electrical behaviour is a constraint and not an advantage. The published constants below are the ones that come up in an enquiry, and the resistivity row is the one that is most often misread.

Read the two resistivity rows together and a claim that appears in a great deal of published material about this wire falls apart. Between 20 °C and 100 °C the resistivity rises from 0.096 to 0.130 micro-ohm metre, which is a change of about 35 % over eighty kelvin, or roughly 0.44 % per kelvin averaged over that interval. The resistance of nickel wire is therefore strongly temperature dependent and it keeps rising above 100 °C, which means that a statement that the wire holds a stable resistance value at high temperature is the opposite of what the published data shows.

The second point follows from the same table. Nickel is a moderate conductor and its resistivity is several times that of copper, so it is not specified into a part for conduction efficiency. What it is specified for is the pair of properties in the rest of the table and in the corrosion section: a stable surface and a defined behaviour at temperature. A designer who wants conductivity reaches for copper, and a designer who wants a resistance that does not move reaches for an alloy designed for that purpose.

This is the largest group of uses for the wire, and the parts are unglamorous: connecting wires, leads for electrical equipment, terminals, and the contact parts of relays and switches. Nickel appears in them because the surface stays clean and the contact resistance stays predictable in a damp or mildly corrosive atmosphere, which is a different requirement from high conductivity. The published description of the material places it in chemical processing and electronics for exactly that reason.

Two qualifications belong with the group. In relay and switch contacts, nickel is a candidate where the duty is a light load over a long life in a corrosive atmosphere, and the heavy arcing and mechanical wear classes belong to a different family of contact materials. In high-frequency and high-power assemblies, the conductor itself is normally copper or a plated metal, and the nickel parts are the leads and supports that have to hold their shape and their surface at temperature.

This is the application where the wire and strip of this material are bought in the largest volume, and the polarity rule is the part that is most useful to have on paper. In a lithium-ion cell the positive electrode is built on an aluminium current collector and the negative electrode on copper, and the tab material follows the same split, so a pack is assembled with aluminium tabs on the positive side and pure nickel or nickel-plated copper on the negative side. Published battery tab literature states the arrangement in those terms, and the reason is the electrochemical compatibility of tab and current collector and not a preference between two metals.

The welding point is the second reason this material is chosen here, and it follows from the resistivity and not in spite of it. Pure nickel is described in battery assembly literature as the easiest of the tab materials to spot weld, because its resistivity sits in the range that resistance welding needs in order to generate heat at the joint. A pack built with nickel interconnects can therefore be welded with the standard equipment used for cell assembly, which is a production decision and not a materials one. In nickel-metal hydride and nickel-cadmium packs the same wire is used for the interconnects and the terminal connections, and in lithium-ion packs, including the 3C and power classes, it is the tab and interconnect material on the negative side.

Where the wire is used as a connection and not as a conductor of power, the requirement is a joint that does not corrode and does not come loose over thousands of cycles. That is a corrosion and a mechanical requirement together, and it is the reason the material survives in packs where a cheaper metal would not.

This is the group where the published data contradicts the way the material is usually described, and the contradiction is worth stating plainly because it is the difference between a heater that works and one that fails. Two claims have to be separated. The first is that nickel wire holds a stable resistance at temperature, which the resistivity table above rules out, since the value rises by about 35 % between 20 °C and 100 °C and continues to rise. An element whose own resistance moves by that much does not deliver a predictable power for a given voltage, and element design assumes a resistance it can calculate across the working range.

The second claim is that the wire does not oxidise over long periods at high temperature. The material bulletin answers that one directly, and the answer is a warning: heating in oxidising atmospheres at high temperatures should be avoided because of the danger of intergranular oxidation. Intergranular oxidation is not surface discolouration. It runs along the grain boundaries into the metal, and a wire that has been through it fails at the boundary and not at its surface. An element wire operating in air at furnace temperature is the exact duty the bulletin tells a user to avoid.

The elements specified for furnace and appliance duty are therefore drawn from the chromium-bearing heater alloys, which form a protective oxide at temperature and whose resistance behaviour is the one an element design is built around. Nickel wire is still a resistance material in the narrower sense of the phrase, and the duties where it belongs are the moderate ones: a controlled atmosphere, a lower working temperature, or a component where the resistance is not the thing being controlled.

What survives from the usual description is the processing side. The wire is ductile in the annealed condition, which the bulletin states as low hardness and good ductility, and it can be wound into a coil or formed into a mesh without cracking. That formability is real, and it is the reason the wire appears in wound components of many kinds, including the ones above.

Nickel wire is often described as a thermocouple material, and the standards do not support that. The standardised base-metal and noble-metal couples are built from alloys and from defined pairs: nickel-chromium against nickel-aluminium in a Type K, nickel-chromium-silicon against nickel-silicon in a Type N, nickel-chromium against copper-nickel in a Type E, iron against copper-nickel in a Type J, and copper against copper-nickel in a Type T. Pure nickel is not one of those legs. A wire used as a measuring leg outside those defined pairs does not produce an output that any published table describes, so a thermocouple is not an application this material can be sold into on the basis of a standard.

The real role of nickel in temperature measurement is the resistance thermometer, and it is a real one with a published limit. Nickel is one of the metals used for resistance sensing elements, and the nickel element conforming to the DIN 43760 requirements is limited to a measurement range of −80 to +260 °C. That ceiling is well below the range of the platinum element used in the same instruments, which is the industry standard for the duty, and the reason is the same curve that appears in the resistivity table: nickel is a sensitive sensing metal and a non-linear one, and the non-linearity is what caps its useful range.

So a wire that is bought for a temperature-sensing application should be ordered against the resistance thermometer specification and its element design, and not against a thermocouple type number, because the two are different instruments with different materials.

Each row below is a duty the published data qualifies or rules out, and the qualification is part of what the wire is.

Two things, and they are different. The wrought grades are described in their bulletin as commercially pure at 99.6 %, while the specification they are sold to sets a minimum of 99.0 % for nickel plus cobalt with the balance in controlled residuals. The certificate is checked against the 99.0 % floor, so the order should carry the UNS number, with the commercial purity figure sitting alongside it as a description.

No, and this is the correction that matters most for a wire going into a resistance or heating duty. The published resistivity is 0.096 micro-ohm metre at 20 °C and 0.130 at 100 °C, a rise of about 35 % over eighty kelvin, and it continues to rise above that. The coefficient averaged over that interval is roughly 0.44 % per kelvin, so the resistance of a nickel element is strongly temperature dependent.

Not in air at high temperature, on the evidence of the material's own bulletin, which states that heating in oxidising atmospheres at high temperatures should be avoided because of the danger of intergranular oxidation. A nickel element can be used at a lower working temperature or in a controlled atmosphere where the resistance change is acceptable, and the elements specified for furnace and appliance duty are drawn from the chromium-bearing heater alloys.

Not as a standardised leg. The defined couples are built from alloy pairs, such as nickel-chromium against nickel-aluminium in Type K and nickel-chromium against copper-nickel in Type E, and pure nickel is not one of them. The temperature-measurement role of this material is the resistance element, where the nickel version conforming to DIN 43760 is limited to −80 to +260 °C against the much wider range of the platinum element.

On the negative side. A lithium-ion cell is built with an aluminium current collector on the positive electrode and copper on the negative, and the tab material follows that split, so packs are assembled with aluminium tabs on the positive side and pure nickel or nickel-plated copper on the negative side. The welding behaviour of pure nickel, which published battery assembly literature describes as the easiest of the tab materials to spot weld, is the reason it holds that place in production as well as in design.

The wire is supplied in coils or on spools, in the annealed or the cold drawn condition, from 0.025 mm diameter upwards and to drawing. The material standard for wire of this family is ISO 9724, with ASTM B 160 and its ASME equivalent covering rod and bar and ISO 9723 covering bar, and the analysis and mechanical results for the lot are issued on the inspection document for the heat the wire came from.

Pure nickel wire is a two-grade family, separated by carbon and identified by UNS number, drawn from commercially pure nickel at a nominal 99.6 % against a specification minimum of 99.0 % for nickel plus cobalt. It is bought for its surface and for its behaviour in reducing chemicals and in caustic alkalies, and not for its conductivity or for a constant resistance, because its resistivity rises by roughly a third between 20 °C and 100 °C. The two applications where the published data has to be read carefully are the heating element, where the material bulletin warns against heating in oxidising atmospheres at high temperature because of intergranular oxidation, and the thermocouple, where the standardised legs are alloys and the real measurement role of this material is the DIN 43760 nickel resistance element with a range to 260 °C.

Where the wire is bought in the largest volume is the battery pack, and the polarity rule is the useful part of that. Aluminium tabs on the positive side, pure nickel or nickel-plated copper on the negative side, and nickel strip or wire for the interconnects, chosen because it spot welds reliably and because the joint holds its surface over the life of the pack. The wire is supplied in coils and on spools from Changzhou, Jiangsu in the annealed or cold drawn condition, cut to length or continuous, with the analysis and mechanical results issued against the heat.

Special Metals Corporation bulletin for Nickel 200 and Nickel 201. Source for the description of the material as commercially pure 99.6 % wrought nickel, the composition limits including the 99.0 % minimum for nickel plus cobalt and the carbon limits of 0.15 % and 0.02 %, the designations and Werkstoff numbers, the physical constants including the density, the melting range, the Curie temperature, the modulus of elasticity and the thermal conductivity, the electrical resistivity of 0.096 micro-ohm metre at 20 °C and 0.130 at 100 °C, the statement that annealed nickel has low hardness and good ductility, the description of the material as highly resistant to reducing chemicals and unexcelled in resistance to caustic alkalies, the warning that heating in oxidising atmospheres at high temperatures should be avoided because of the danger of intergranular oxidation, the report that prolonged exposure between 800 °F and 1,200 °F precipitates graphite, and the list of specification numbers by product form.

ISO 9723, nickel and nickel alloy bars; ISO 9724, nickel and nickel alloy wire and drawing stock; ISO 9725, nickel and nickel alloy forgings; with ASTM B 160 and ASME SB 160 for rod and bar. Source for the standards that cover the forms supplied and for the statement that the material standard for wire of this family is ISO 9724, which specifies composition, tensile properties, surface quality and dimensional tolerances for wire.

Published battery tab and pack assembly literature, including supplier technical pages for lithium battery tabs and for pure nickel strip in spot welding. Source for the statement that the positive electrode of a lithium-ion battery uses aluminium tabs and the negative electrode uses pure nickel tabs or nickel-plated copper tabs, and for the statement that pure nickel is the easiest of the tab materials to spot weld. These are commercial technical sources and are quoted as such and not as standards.

Published instrumentation literature on resistance elements, including the statement that nickel is limited to a measurement range of −80 to +260 °C and that nickel elements conform to the DIN 43760 requirements, together with the statement that platinum is the industry standard sensing material with a much wider range. Source for the comparison of the two sensing materials.

Published thermocouple material and type listings. Source for the leg compositions of the standardised base-metal couples, including nickel-chromium against nickel-aluminium in Type K, nickel-chromium-silicon against nickel-silicon in Type N, nickel-chromium against copper-nickel in Type E, iron against copper-nickel in Type J, and copper against copper-nickel in Type T, and therefore for the statement that pure nickel is not one of the defined legs.

Product data held by DLX Alloy. Source for the forms supplied, the diameter range from 0.025 mm, the annealed and cold drawn conditions, the coil and spool packing, and the inspection position.

Every figure above is a published value for the material named, and the coefficient quoted in the resistance discussion is calculated from the two published resistivity values and not measured on a delivered wire. A nickel wire lot is accepted against the ordered grade, diameter, temper, and surface condition, with the analysis and mechanical results confirmed on the inspection document for the heat. Resistivity, hardness, and tensile properties move with the grade, with the temper and with the wire diameter, which is why the condition is stated wherever a value is quoted. Applications outside the duties described here should be checked against the specific medium and temperature, and the two qualifications on this page, the oxidation warning for heating duty and the measurement range of the nickel element, are the ones to carry into that check.