TUBACEX is reinforcing its grade portfolio to produce high nickel alloys, as series 200, 400 and 600. The article below describes the advantages of using Alloy 400 in Hydrofluoric-based Alkylation units in refineries, which we already supply to customers.
Use of Alloy 400 against Hydrofluoric Acid Corrosion in Alkylation Unit of Refinery
Alkylation units in a refinery produce a high octane component for gasoline blending where isobutane is reacted with various olefin feeds to form alkylate (isoparaffin). Hydrofluoric acid (HF) is normally the catalyst used for chemical reaction between isobutene and olefin feeds. The alkylation unit is often subjected to corrosion-related material failure due to this acid.
TUBACEX, as a major stainless steel seamless tube and pipe manufacturer, has focused on developing new materials as well as carrying out metallurgy improvements in stainless steel to prevent corrosion failures in presence of HF. Besides metallurgy, which is of course critical, there are several other aspects in tube and pipe production such as heat treatment which play a major role in defining the mechanical/corrosion properties of steel. Apart from carrying out improvements in the production technology/process and steel metallurgy, TUBACEX is also developing other alternative ways of increasing material life such as ceramic coating solutions.
Overall carbon steel with low impurity levels is the main choice for low temperature (< 50ºC) and high HF concentration in an alkylation unit. However corrosion rates in carbon steel vary depending on inclusions/impurities. Moreover, life expectancy may not be achieved as material quality varies according to different suppliers. In addition, risks of Hydrogen-induced cracking and hydrogen blisters are highly dependent on the cleanliness of carbon steel.
Alloy 400 material can be used in some of the most corrosive atmospheres such
as acid regeneration column and pre-heaters.
Alloy 400 presents advantages compared to carbon steels. It has high strength and toughness over a wide temperature range and excellent resistance to many corrosive environments. The use of nickel-alloyed steels is now common in hydrofluoric acid environments as most of these alloys form a thin protective metallic fluoride layer. It has excellent corrosion resistance to HF and is less affected by acid concentration, temperature and velocity unlike carbon steel.
This material can be used in some of the most corrosive atmospheres such as acid regeneration column and pre-heaters. Current tests conducted on the three most corrosive sections, i.e. pre-heaters, regeneration column and HF stripper, by the Nickel Development Institute have shown that Alloy 400 gives the best results followed by Alloy 600 and Ni 200/201. The long term experience of refineries having alkylation units has shown similar results.
However, Alloy 400 has a tendency for transgranular and/or intergranular corrosion, especially in the presence of oxygen. Hence if oxygen cannot be controlled in the process environment then proper stress relief of tube/pipe (and also other components) becomes very important and consequently the production systems and quality controls carried out by Tubacex play a major role.