The Source of Hydrogen in Steels
Steel is one of the most commonly used materials in the world, with its properties and uses that are applicable in many industries like construction, maritime and automotive. But steel is a complex material, we have to take into account elemental composition and a further sub-integral characterised by microstructure and grain size, among other parameters. In order to understand the effects of the different alloying elements, we have to analyse how they interact. Elements like Carbon, Manganese and Phosphorus, although crucial for steel performance, don’t form any significant compound with it. On the other hand, it is the case with some particular elements, like Hydrogen, that can exist in different chemical forms in the steel microstructure by interacting with other alloying elements like, for example, Nickel and Chromium.
Hydrogen can be incorporated into steel through different pathways, while the most significant and frequent ones take place during steelmaking operations. The main sources of Hydrogen can be identified as:
• Hydrogen coming from raw materials. Iron Ore, Inoculants and Scrap may be present in some extent of Hydrogen concentrated in them, related to the material composition and the purity of the same. Depending on the raw material and the steelmaking process, the amount of Hydrogen can vary from a few ppm up to %wt.
• Hydrogen from the reduction process. When reducing from iron ore to iron, a high amount of hydrogen is evolved. Nowadays, this part of the process is carried out in a direct reduction process, which prevents this hydrogen from the reduction process to be incorporated into the steel melt. Even so, some part of the evolved hydrogen might get in contact with the steel, increasing the hydrogen concentration.
• Hydrogen from refining processes. Refining is the process in which the chemical composition of the steel increases its precision and homogeneity, by means of chemical reactions and gases additions in order to reduce impurities and restore the balance of the steel chemistries. The main process employed to enrich steel with some elements is the vacuum treatment process and can cause the presence of dissolved hydrogen ranging from few ppm up to 1000 ppm.
• Hydrogen from the welded zone During welding operations, both heat flux and flux employed in case of gas welding can cause the increase of hydrogen in the welded zone, floating around the wires and the electrodes and incorporated into the molten metal. This hydrogen can come from moisture of the electrodes or from the atmosphere. Depending on the process, the amount of hydrogen present can range from 25 up to 1000 ppm.
• Hydrogen from pickling processes. This is probably the highest source of current incorporated hydrogen. Once the welding process is complete, and in case that the steel is contaminated because of oils, grease or just oxidised layers, these have to be removed in a process known as metal pickling. One of the common reagents used in pickling is hot phosphoric acid, which can dissolve the contamination and damage itself in order to get rid of scaling, rust and mill scale. Its activity not just towards the oxidised impurities but also can dissolve hydrogen and dulcolcially incorporate it into the steel, increasing the hydrogen variability up to few thousands ppm.
The concentration of hydrogen registered in steels throughout the steelmaking chain can be huge and, in case that its presence is not properly taken into account and controlled, it can present serious defects, compromising the ultimate performance in service. This is the reason why monitoring and controlling the sources, in order to be aware of the hydrogen content at each process, is so important to guarantee the quality of the final steel product and its use in applications like shipbuilding or automotive industries.
Hydrogen can cause serious defects in any steel, like embrittlement and cracking of the material, reducing its ductility and, consequently, its use in many applications. This can cause serious economical losses, making monitoring and controlling the source of hydrogen a key factor for the sustainability of the steel business.
