Nichrome- Corrosion resistance at high temperature

In the matrix of nichrome, chromium is fully miscible in the nickel. It is maximum at 47% in the eutectic temperature and decreases at content of 30% at room temperature. Different nichrome alloys offer supreme resistance t high temperature oxidation and extreme corrosion media and offer supreme wear resistance.

Oxidation resistance

The pair of nickel and chromium widely improves the potential of nichrome to oxidation. It is because of enhanced diffusion rate of oxygen. This process opposes the increased chromium content about 30%. Increased chromium content alters the process. The oxidation resistance provided by Nichrome improves with increasing concentration of silicon, cerium, calcium and zirconium. The oxide layer developed is a pair of nickel and chromium oxides.

Nichrome Heating alloy provides major improvement in electric resistivity by enhancing concentration of chromium. Inclusion of chromium widely improves electric resistance for use in the heating equipments. The nichrome provides outstanding electric characteristics with good strength and ductility therefore it is suitable for wire drawing. The commercially used Nichrome alloys are ideal for various industrial operations. Nominal chemistry changes are made to achieve the specific set of properties. The change in chemistry does not create noticeable effect on the mechanical characteristics of Nichrome grades. Large concentration of active elements prevent scale flaking during cyclic heating and cooling. It rarely becomes an issue while the regular use of heating metals so the extra elements are nominally included.

Nichrome 80 is used as the heating element for offering high temperature service about 1100oC and is also used in thermocoupling that actively drifts in the region of temperature about 1000oC due to oxidation after prolong use. This influence can be handled with the inclusion of silicon.

Corrosion resistance at the elevated temperature

Nichrome heating element is used for wrought and cast elements in the high temperature services as it provides greater resistance to oxidation and corrosion unlike to FeCrAl elements. Nichrome is best fit for use in the oxidizing media.

In the conditions containing sulfur, nichrome develops chromium sulfide. The development of nickel sulfide is recommended instead chromium sulfide as it resists the formation of nickel-nickel sulfide eutectics with low melting point. Eventually in the presence of sulfur, nickel interacts with sulfur to create low melting point eutectic solutions that result into liquid phase cracking.

The alloys that are degraded get wart development on their top layer. This corrosion can be eliminated by chromium sulfides containing high chromium content.

Development of Nichrome Dew Heaters

Developing nichrome dew heaters includes assessing the required resistance and use parallel lengths of nichrome wire to receive it. Follow the instructions to handle low resistant values therefore it is significant to eliminate the resistance in the leads from the meter values. Check the meter leads simultaneously to calculate the lead resistance. Now eliminate this lead resistance from each measurement made in the following guidelines. For instance, if touching the leads includes 0.5 ohms, a piece of Nichrome wire reading 15 ohms is practically only 14.5 ohms.

a.      Calculate the circumference that you wish your heater to install in inches. For instance C11 describes 38 inches.

b.      Divide 190 with circumference in inches to find the required heater resistance such as 190/38 inches = 5 ohms. 

Find the least length that any piece of Nichrome wire is possible to cut. If Nichrome is smaller than the least value, it has very small resistance that large current travels through it to overheat the wire. The longer wire shows high resistance so small current travels through it and heat magnitude is decreased.

Without slicing the Nichrome wire, extend it and use alligator clips or other means of momentarily applying 12 Volts to the complete wire length. Considering that nichrome hardly gets warm, move single alligator clip a few inch closer to decrease the wire part the 12 Volts is being applied. Moving it closer as long as a point is found that is hot however not much to melt anything that touches the wire. In the above instance, 30 inches was the smallest piece found that could be used without becoming very hot. Melting of heater strip by wire is not required. The smaller piece of wire can be cut to be hotter that will get as the smaller wire shows small resistance hence allows more current to travel through it.

Disconnect power from heating element Nichrome 80 wire and check the meter leads on the wire where the clips were positioned to assess the resistance the smallest wire length. Here for 30 inches of wire, the resistance value found was 14 ohms.

Nichrome’s Applications

Do you need resistance wire in your manufacturing applications? Contact Heanjia, a leading producer of nichrome heating elements. This non-magnetic material offers high electrical resistivity and excellent resistance to oxidation at the elevated temperatures.  

It is used as bridgewire in explosives, support wire in kilns and also as a heating element in the hair dryer, this heating element offers supreme service in your applications.

Nichrome wire is delivered in the various forms such as coils, spools etc. It is introduced in two grades- Nichrome 60 and Nichrome 80. The wire is made in the different sizes to meet customer’s requirements. The commonly made forms are round wire or square wire.

Uses

Nichrome has special set of characteristics for example high electrical resistivity, good oxidation resistance and prevention of corrosion and high melting temperature. Therefore it is used in several applications that need high heat supply for example explosives, household equipments, arts and crafts and as heating elements. It is commonly used in hair dryer, ovens, kilns, toasters and custom wires. Nichrome also finds its set of applications in explosives and fireworks and in electrical ignition units. Various electrical matches and rocket igniters utilize nichrome wire.

Nichrome is also used in art industry. Several types of ceramics include use of heat resistant nichrome wire for internal support when fired in kiln. The metal withstands up to 1400oC without causing problem of heat involve during ceramic firing.

If you are not sure about the perfect alloy for your application, get assistance of our experts. They will work with you to ensure that they are meeting your requirements.

How much resistance is required for a heating element?

Generally it is assumed that a heating element requires high resistance, as resistance is responsible for heat production. But it is not the complete truth. What develops heat is the current going through the element instead the amount of resistance offered. High current value through the metal is more crucial than driving that current to face high resistance. It is confusing and counter-spontaneous however it is very easy to observe why it is valid.

Consider that you create resistance for your heating element as large as feasible. Following Ohm’s law, you find the current travelling through the metal would be very small. You may have large resistance, but without current, the heat doesn’t produce.

So just go reverse, and make resistance nominal. Here you will find different situation. However current I can be very high, resistance would become virtually zero. Therefore the current would be included into the element like a train that has no stoppage and doesn’t develop any heat.

Therefore a heating element is needed that has balance among these both factors. Sufficient resistance is required to develop heat however not as it decreases the current widely. Nichrome is a supreme selection. The resistance offered by it hundred times more than the same size copper wire however only 1/4^th of graphite rod. The numbers show themselves that nichrome is a medium conduct with sufficient resistance however not an insulator.

The power taken or consumed by current supply is equal to the voltage times of current. Heat is directly proportional to resistance, however also proportional to current square as P = I²R. Therefore the current has more effect than resistance on the heat production. Double resistance doubles the power but double current increases heat production by four times. So current has the major role. Measuring the resistance is easy in a traditional lamp.

In few cases, the heating elements can be easily seen for example in a toaster that shows nichrome ribbons installed in its walls as they shine red hot when are in service.

Electric radiators develop heat with burning red bars, on the other hand, electric convector heaters

normally have concentric, circular heating elements located in front of the electric fans. Few equipments have visible elements that serve at lower temperatures and do not blaze for example electric kettles that do not need to serve beyond the water boiling point. Various applications include heating elements that are fully hidden to ensure the user’s security. Similarly in the electric showers and hair curling tongs, the heating elements are completely hidden to prevent any risk of electrocution.

This makes heating materials to work in a simple and easy manner however there are several factors that are considered by electric engineers while designing the heating equipments such as voltage, current, length and diameter of element, material type and temperature. Various other factors considered are coiled element built from round wire, wire diameter and coil forms such as diameter, length, pitch etc, these also have a significant effect on the performance on the material performance.

If you take a ribbon, its thickness, width, surface area and weight should also be considered.

Different heating devices for example electric furnace, oven, heaters etc consumes electric energy to develop heat. In these systems, heating material transforms the electric energy into the heat energy. The service material offers the heating effect depending on the current supplied. Significant factors that influence the resistivity of heating elements are:

Temperature, alloy, mechanical stress, age hardening and cold processing.

Temperature: The electric resistivity in Nichrome 80 strip varies with temperature. It generally increases by raising the temperature. The variation in resistivity of a material with variation in temperature follows a formula. Metal ‘ resistivity is in proportion to the temperature that raising temperature increases the resistivity value of the metal. The metals have positive temperature coefficient of resistance. Most of metallic materials attain zero resistivity at absolute zero temperature. This mechanism is named as superconductivity. In case of semiconductors and insulators, the resistivity decreases by raising the temperature. So we can say that semiconductors and insulators keep negative temperature coefficient of resistance.

Alloy: Alloy is a combination of two or more metals. Allowing is done to receive a desired set of mechanical and electrical characteristics. Electric resistivity of alloy steeply raises with addition of alloy concentration.  Little concentration of contaminants widely increases the resistivity value. In fact a contaminant of small resistivity value also improves the resistivity value of base metal noticeably.

Mechanical Stress: Stress on the crystal structure of a heating element creates localized strain in its structure. These strains affect the motion of free electrons across the material that results into improving material’s resistivity. Thereafter annealing of metal decreases the resistivity value of the material. Annealing eliminates the mechanical stress on the material that removes the localized strains among its crystal structure. It results into decreasing the resistivity. For instance, the resistivity of hard drawn copper is higher than annealed copper.

Age hardening is a heat processing of material that is performed to enhance its yield strength and to create an ability to prevent the lasting deformation due to external factors. It is also named as precipitation hardening that improves strength in materials by the development of solid contaminants. Such solid contaminants affect the crystal structure of metal that changes the motion of free electrons in the metal hence increases its resistivity value.

Cold Processing: It is a part of manufacturing process that is significant in increasing the strength of metallic or alloy materials. Cold Processing is also called as work hardening or strain hardening that adds the desired mechanical strength in the metallic material. Cold processing deforms the crystalline structure of the material by affecting the motion of random electrons in the material that results into increasing the resistivity value in metal.