TPi March 2020

Tube corrosion in high temperature applications

with manufacturers can go a long way to sharing the burden and getting the best result. In recent years budgets have continued to be squeezed and those responsible for selecting equipment and materials have found themselves having to perform a tricky balancing act of finding the right solution but at the same time, the right price. As in most situations going for the cheapest option will undoubtedly cause problems in the long run. Making the decision to go for a more expensive, high performance material may be perceived as something of a ‘leap of faith’ but in the long term this will pay dividends. Today much of the emphasis is on keeping any downtime to a minimum and this is achieved by avoiding unforeseen maintenance requirements and shutdowns, which can be so disruptive to production however short. There are a number of types of corrosion occurring in high temperature applications and these are oxidation, carburising, sulfidation – including oxidising sulphur and reducing sulphur – and nitridation. Oxidation occurs when steel is heating in air and an oxide layer or scale will form on the surface. Carburisation happens when carbide forms from a reaction of the metal to hot gases which contain C compounds or hydrocarbons (such as methane, ethane and propane). Sulfidation is caused by a reaction of metals with hot gases containing H 2 S and SO 2 . Nitridation is where hard and brittle nitrides form when the steel comes into contact with gaseous mixtures containing N 2 , combination of N 2 and H 2 . Flue gas corrosion is another issue which is due to by-products from the combustion process using clean fuel (ie natural gas), fuel oil and coal with a sulphur content. It is possible to increase the lifetime of lances while also boosting recuperator efficiency by selecting high performance material. Across the globe, the blast furnace is still a widely used manufacturing process and sometimes replacement of PCI lances can be as often as every few weeks. In addition to being costly, the frequency of shutdown will not only be disruptive to production, but will also bring the risk of safety into the operational equation. Lance lifetime is just one area where a better grade selection can make a big difference. For example, when working with a Brazilian steel mill, the lifetime of its blast furnace lances was improved by 300 per cent by switching to Sandvik 253 MA in pulverised coal injection lances. The payback time for the customer was less than one year and in addition they found they had also reduced their maintenance costs while at the same time boosted their pig-iron productivity. In a 21 st Century

Corrosion in high temperature applications is always a threat that can lead to unplanned shutdowns and in turn cause a detrimental effect on productivity. Selecting the right material for high temperature (HT) applications will inevitably make a significant contribution to avoiding these issues and to improving the bottom line. Some of these applications include kiln lances, pulverise coal injection lances in blast furnaces, recuperators, muffle tubes and range of other processes. Here Alessandra Spaghetti of Sandvik looks at the type of issues that can occur in such situations and how to avoid problems, while offering advice on how to get the material specification right at the project or production system design stage as well as providing support during unplanned maintenance by discussing the best material upgrade. High temperature corrosion occurs in applications where the temperature is above 500°C. This so called ‘cross over temperature’ brings the material into different properties compared to those at room temperature. This means that consideration has to be given to creep strength instead of yield strength, hot corrosion instead of wet corrosion mechanism and structural stability. At temperatures of above 500°C, the gases start diffusing into the metals. The most common are: O 2 , CO, CO 2 , NO 2 , H 2 S, SO 2 , but in some applications molten salts are also found. Selecting the right material for a high temperature application can and will make significant difference to the bottom line. There are many situations where this principle applies including the replacement of lime kiln lances or handling of the heat in a steel furnace. Recuperator, muffle tubes, mineral insulated cables and pigtails are just some of the types of processes discussed in this article. There are ways to avoid such high temperature corrosion issues that can lead to unplanned shutdowns, which can prove to be costly and disruptive, and instead go a long way to actually boosting productivity. A good place to start is to evaluate and examine the individual application, operating environment and the type of challenges the situation is presenting. This could be issues such as carburisation, creep deformation as the tube material being used reaches its cross over temperature, or embrittlement in the range of 600°C to 850°C (the range of temperatures and kinetics of the formation depend on the grade) as the secondary phase (sigma, gamma phase) formation/ precipitation is initiated. Selecting a material that will provide mechanical strength, structural stability and the right level of resistance to corrosion will be critical factors at the outset. At the same time, reducing maintenance requirements and lifecycle costs are also important considerations and working

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TUBE PRODUCTS INTERNATIONAL March 2020

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