Question

Which of the following would be modeled by first law diffusion? a. Addition of Nitrogen to a gear to increase hardness. b. Loss of helium through the walls of a rubber balloon. c. Carbon diffusion into the furnace atmosphere during heating of test specimens in a laboratory oven.

Answer 1

Which of the following would be modeled by first law diffusion?

a. Addition of Nitrogen to a gear to increase hardness.

b. Loss of helium through the walls of a rubber balloon.

c. Carbon diffusion into the furnace atmosphere during heating of test specimens in a laboratory oven.

Answer)

First Law of diffusion can be explain as follows,

Diffusion can be described as the random movement of particles through space, usually due to a concentration gradient. Diffusion is a spontaneous process and is a result of the random thermal motions between two particles.

First Law of diffusion can be given as following equation,

J= -D(dc/dt)

J is the flux and is defined by the number or particles that are moving past a given region divided by the area of that region multiplied by the time interval. Unit for J are m^​-2 s^-1.

The letter DD represents the diffusion coefficient with units m²s.

c is the concentration of the gradient with units molecules m^-3.

Equation indicates that if the flux and the change in the concentration over time are known, then the diffusion coefficient can be calculated. The negative sign indicates that the concentration gradient is negative. The first law can only be applied to systems in which the conditions remain the same— in other words, if the flux coming into the system equals the flux going out. Fick’s second law is more applicable to physical science and other systems that are changing. This second law is applied to systems in which the condition are not steady, or the solution in not equal throughout.

Answer a)

Addition of Nitrogen to a gear to increase hardness can be modelled by first law of diffusion.Duplex Stainless Steels (DSS) have excellent corrosion resistance properties and poor wear resistance. Plasma nitriding is used to increase the surface hardness, by nitrogen diffusion into the crystal lattice, and to improve wear resistance. DSS samples were plasma nitrided at temperatures between 350 °C and 500 °C for 240 minutes under an atmosphere of 75% N2 + 25% H2. Nitrogen diffusion in both phases appears to be similar. The layer thickness on the alpha phase is greater than in the gamma phase. Nevertheless, the difference on the layer thickness in both of the phases is small. From these results, it was proposed a model of nitrogen diffusion into a two-phase stainless steel to explain the morphology of the interface between the layer and the substrate. Duplex stainless steels (DSS) are used in applications where they must have good wear resistance properties, having a good resistance to corrosion. To accomplish this, DSS are submitted to thermochemical treatments as plasma nitriding to increase the surface hardness, wear resistance and fatigue properties. Nitriding is done at low temperatures to prevent nitrides formation, which reduces corrosion resistance. Hardening is given by the nitrogen diffusion into the crystal lattice (Reference: On the Nitrogen Diffusion in a Duplex Stainless Steel, material research; Margarita Bobadilla^, Andre Tschiptschin).

Answer b)

Loss of helium through the walls of a rubber balloon can also be modelled by first law of diffusion because you must have observed that balloons inflated with helium gas rises in the air on the first day but on the next day they fall down like ordinary balloon filled with air this is because helium in balloon slowly licks out through walls of rubber balloon while air diffuses in.

Answer c)

Carbon diffusion into the furnace atmosphere during heating of test specimens in a laboratory oven.

Gas carburizing is an important heat treatment process used for steel surface hardening of automotive and aerospace components. The quality of the carburized parts is determined by the hardness and the case depth required for a particular application. Mass transfer takes place during carburizing process, carbon diffuses in the still/glass in the furnace atmosphere. (reference: Fundamentals of Mass Transfer in Gas Carburizing; Olga Karabelchtchikova)