HASTELLOY HYBRID-BC1 All-purpose weapon in the battle against corrosion
Aggressive process media with wide-ranging properties present plant manufacturers with enormous challenges. The HASTELLOY HYBRID-BC1 hybrid alloy offers a solution.
The term “corrosion” has its roots in the Latin corrodere, meaning “dissolve, erode, eat away”. In the technical sense, “corrosion” refers to the reaction of a material with its ambient elements that results in measurable change to the material and may lead to impaired functioning of a component or system. Corrosion damage is a serious economic factor; destructive chemical processes are a threat to steel constructions such as bridges or girders, and represent a costly problem for industrial installations, power plants and pipelines – to name but a few examples. The World Corrosion Organization (WCO) estimates the economic damage caused by corrosion at the sum of US-$ 2.2 trillion per year.
Of course there are materials that are resistant to the advance of corrosion and are thus elevated to the status of “corrosion-resistant”. Classic stainless steels, for example, offer a useful basic level of corrosion protection – but soon reach their limits when faced with aggressive substances like inorganic acids or chloride environments.
Corrosion is determined not only by the process fluids
The limits of endurance of these steels are demonstrated by their susceptibility to stress corrosion cracking, corrosion pitting and crevice corrosion. Nickel puts up a better defence; pure nickel is the material of choice for applications involving process fluids with high levels of sodium and potassium hydroxide (caustic chemicals).
Zirconium and titanium alloys offer roughly the same level of protection as corrosion-resistant nickel-based alloys; however, as these reactive alloys have a variety of highly specific areas of use and are intolerant to specific ionic substances, they will not be examined further in this article.
Nickel-based materials thus remain as the materials with the broadest effective range, resisting most forms of corrosion. They are extremely versatile and are resistant to both oxidizing and reducing acids and alkalis, as well as minimizing the problems outlined above such as stress corrosion cracking, corrosion pitting and crevice corrosion. Nickel-based alloys are also extremely ductile, weldable and formable and the production of industrial components and fabrications is a relatively straight forward process.
Sulphuric acid, for example, is a highly aggressive fluid that may occur in both reducing and oxidizing forms. This is where nickel alloys such as Hastelloy B and C type alloys from Haynes International, Inc., come in. Based in Kokomo, Indiana, Haynes International is a global leading developer and manufacturer of nickel and cobalt alloys and has been a partner of Zapp Group for over 60 years. B type alloys comprise nickel-molybdenum alloys, while C-type alloys include nickel-chromium-molybdenum alloys.
However, the corrosion process is determined not only by the process fluids themselves; contamination of the fluids represents a further factor that is often difficult or impossible to influence or predict. In practice, consideration of this factor to minimize plant or system corrosion would require different materials to be used for one and the same acid at different degrees of contamination.
Tests involving boiling 10% and 50% sulphuric acid show the extent to which the two alloy types B and C are suitable for individual applications of pure and contaminated sulphuric acid respectively. HASTELLOY B-3 alloy with approx. 30% molybdenum and approx. 1.5% chromium shows outstanding corrosion resistance to pure reducing sulphuric acid in the two concentrations given, with negligible corrosion rates. However, as soon as even trace amounts of ferric ions are added to the acid, corrosion resistance decreases – dramatically in some cases. The reverse can be seen in the curve for 50% sulphuric acid, where the corrosion resistance of HASTELLOY C-2000 alloy with 23% chromium and 16% molybdenum increases in parallel to a lower increase in the proportion of ferric ions. Ferric ions can often be found in process streams as a contaminant due to the corrosion of iron containing alloys further upstream.
A new challenge to material quality
The problem is a classic case of “either/or”. In practice, the proportion of oxidizing contaminants is frequently variable and the nature of the acid may change from reducing to oxidizing or vice versa. Plant operators face the same problem when process fluids with opposing properties must be processed in rapid succession.
New processes and product characteristics present a continuous stream of new challenges to material quality in plant engineering. To ensure that the field of plant operation both in and beyond the chemical industry retains its value and cost-effectiveness, a material would be needed that could outperform classic B and C alloys, by coping easily with these changes in fluid properties while also minimizing corrosion.
In fact, a material that is significantly more capable of handling both sets of requirements actually exists; this material combining the best of both worlds is a hybrid alloy.
In technology, “hybrid” – a term derived from the Greek for “mixed, of two origins” – is generally used to denote a system combining two technologies. The prefix “hybrid” indicates a whole that is composed of differing types or processes, with the special feature that each of the combined elements represent individual solutions in themselves – just as the materials listed in the example do for specific groups of process fluids – but, when combined, can create new and desirable properties: here, the capability of processing both reducing and oxidizing materials at high temperatures in one and the same installation.
The new hybrid material: the best of two worlds
The new hybrid material – a nickel-based alloy, for the reasons outlined above – is also manufactured by HAYNES. The HASTELLOY HYBRID-BC1 alloy was developed to fill precisely the gap described, for which no truly effective solution had yet been discovered.
A further example of the ability of a single process fluid to change characteristics can be seen in a series of tests using hydrochloric acid. In Figure 3 the x axis shows the corrosion rate in mm/year for a strongly reducing 2.5% solution of hydrochloric acid (HCl) at 121 °C. The Y axis shows the same HCl with the same parameters, but with one difference: purging with oxygen instead of nitrogen causes the acid to have a strongly oxidizing effect on the metal. In this case, HASTELLOY HYBRID-BC1 alloy (shown in the figure as a red dot) offers an excellent compromise by combining the strengths of alloys from the B and C families, thus significantly reducing the failure risk of both these classic material families in many cases.
Summary: HYBRID-BC1 alloy, featuring “hybrid” properties provided by its composition of 22% molybdenum and 15% chromium, is particularly recommended in cases where both reducing and oxidizing process fluids are used in installations. Today’s chemicals industry is subject to growing pressure; plants are required to perform a variety of chemical operations with varying process media, at ever-shortening process intervals and in batch sizes ranging from small to large.
As a result, the diverse extent of applications offered by the materials used to construct these installations is facing increasingly challenging requirements – which can be effectively fulfilled with the new alloy from HAYNES International. Once again, the Zapp Group is able to supply a material offering highly attractive future potential and filling a major market niche for which no ideal solution has previously been available.