Diffusion into steel

Atomic hydrogen, formed from the dissociation of molecular hydrogen, may diffuse into steel surfaces. The hydrogen then recombines into its molecular form and becomes trapped inside the steel lattice. The subsequent pressure buildup results in blisters that eventually crack the steel. 

Molecular hydrogen cannot diffuse into steel and, therefore, it accumulates on the grain boundary. Due to enormous pressures exerted by hydrogen, discontinuous microcracks are formed in the grain boundaries. The metal strength continues to decrease by the pressure by methane or hydrogen until damage occurs. A steel tube in a boiler would burst under the pressure. 

Hydrogen in the molecular form is not capable of diffusion in steel, however, in the atomic form it diffuses into steel. Surface heterogeneities, voids, inclusion and/or grain boundaries are the preferred sites for accumulation of atomic hydrogen. The atomic hydrogen combines to form molecular hydrogen and builds up a high-pressure internally and lowers the fracture stress. 

Selective Carburi ng. In most components, it is desirable to carburize only parts of the surface. To prevent other regions from carburizing, they must be protected. For holes, simple plugs of copper may be used. In some cases, copper plating can be appHed, but diffusion into the steel must be considered, and the copper may have to be machined off later. Coatings (qv), which can be appHed as a paste and then removed after heat treatment, are also available and include copper plating, ceramic coatings, and copper and tin pastes. 

To make martensite in pure iron it has to be cooled very fast at about 10 °C s h Metals can only be cooled at such large rates if they are in the form of thin foils. How, then, can martensite be made in sizeable pieces of 0.8% carbon steel As we saw in the "Teaching Yourself Phase Diagrams" course, a 0.8% carbon steel is a "eutectoid" steel when it is cooled relatively slowly it transforms by diffusion into pearlite (the eutectoid mixture of a + FejC). The eutectoid reaction can only start when the steel has been cooled below 723°C. The nose of the C-curve occurs at = 525°C (Fig. 8.11), about 175°C lower than the nose temperature of perhaps 700°C for pure iron (Fig. 8.5). Diffusion is much slower at 525°C than it is at 700°C. As a result, a cooling rate of 200°C s misses the nose of the 1% curve and produces martensite. 

A piece of plain carbon steel containing 0.2 wt% carbon was case-carburised to give a case depth of 0.3 mm. The carburising was done at a temperature of 1000°C. The Fe-C phase diagram shows that, at this temperature, the iron can dissolve carbon to a maximum concentration of 1.4 wt%. Diffusion of carbon into the steel will almost immediately raise the level of carbon in the steel to a constant value of 1.4 wt% just beneath the surface of the steel. However, the concentration of carbon well below the surface will increase more slowly towards the maximum value of 1.4 wt% because of the time needed for the carbon to diffuse into the interior of the steel. 

By indirect embrittlement (reaction by-product, atomic hydrogen diffusing into the lattice of the steel)... 

In most circumstances the kinetics of this reaction are controlled by the rate at which the hydrogen can diffuse into the underlying steel, and this reaction is essentially in equilibrium. Consequently it is difficult to study the kinetics of this reaction. A particular situation in which this may be very important relates to the conditions at crack tips, where the hydrogen may be transported into the bulk by dislocation motion, giving rise to very high rates of hydrogen entry. 

Although the majority of the hydrogen produced on the cathodic areas is evolved as gas and assists the removal of scale, some of it diffuses into the steel in the atomic form and can render it brittle. With hardened or high-carbon steels the brittleness may be so pronounced that cracks appear during pickling. Austenitic steels, however, are not so subject to embrittlement. 

If the acid contains certain impurities such as arsenic, the arsenic raises the overvoltage for the hydrogen evolution reaction. Consequently, the amount of atomic hydrogen diffusing into the steel, and the brittleness, increase. 

Variable-hardness pipe, with the harder material in the interior, softer toward the exterior, so that any hydrogen that diffused into the interior steel rapidly diffuses outward and escapes... 

The interfacial chemistry of corrosion-induced failure on galvanized steel has been investigated (2) adhesion of a polyurethane coating was not found to involve chemical transformations detectable by XPS, but exposure to Kesternich aging caused zinc diffusion into the coating. Similar results were obtained with an alkyd coating. Adhesion loss was proposed to be due to formation of a weak boundary layer of zinc soaps or water-soluble zinc corrosion products at the paint metal Interface. 

Certain ions and organics in the solution affect H entiy into the metal (FUtt and Bockris, 1991). More is known about this for deposits onto iron and steel than for other metals. Here, for example, cyanide and many organics adsorb and slow down the desorption reaction of H recombination. Consequently, the steady state, 0H, is increased and this tends to accelerate the rate of H diffusion into the metal, resulting eventually in embrittlement. Conversely, anything in the solution that tends to reduce 0H (e.g., NO3 and oxidizers) will reduce the tendency of the H to enter into the metal and cause damage. 

The Sarcina lutea test is the official US Food and Drug Administration (FDA) test for detecting penicillin residues in milk and dairy products (41). In this test, milk samples are placed in stainless steel cylinders on an agar plate seeded with Sarcina lutea ATCC 9341. As milk diffuses into the agar, inhibitors prevent the growth of the organism, causing a zone the width of which is a measure of the antibiotic concentration. The test is sensitive to about 0.006 g/ ml penicillin G, and confirmation of positive results can be performed by the addition of penicillinase. 

Hydrogen is found everywhere in aqueous electrochemistry, and it plays a large part in materials science, often taking part in the mechanism of the breakdown of materials. Unfortunately, the materials that are most used in engineering construction, iron and its alloys (steels), are susceptible to the diffusion into them ofhydrogen, which under certain circumstances will cause a catastrophic loss of strength of the material. 

Hydrogen has a tendency to adsorb and dissociate at material surfaces, the atomic hydrogen then diffuses into the material and causes embrittlement and diffusion. Materials suitable for hydrogen applications are mainly austenitic stainless steel and aluminum alloys [12, 29]. 

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