Materials and corrosion in process equipment: families, cladding and what not to decide upfront

Process equipment frequently uses austenitic stainless steels, duplex and superduplex steels — which combine mechanical strength with behaviour in certain media — and nickel alloys for more demanding services. Each family has fields of application that depend on fluid composition, temperature, pressure and the presence of aggressive species such as chlorides or sulphides.

Cladding consists of applying a layer of resistant material over a base material, usually carbon steel, combining the economy of the base material with the corrosion resistance of the surface layer. It is not an alloy or a homogeneous material: its behaviour depends on bond quality, lining thickness and fabrication and welding processes. ISO 18796-2 addresses aspects of linings; like any standard, its scope and currency must be verified before citing it as a requirement.

In the presence of H2S, for example, there is no default recommendable material: cracking risk depends on concentration, temperature, pH, stress state and other service factors. ISO 15156-1:2020 addresses the selection of cracking-resistant materials for H2S-containing environments in oil and gas production; its scope, current edition and applicability to a specific item must be verified before invoking it.

Material selection and qualification — including welding procedures and testing — belong to materials and corrosion specialists and to the project's competent engineering. Commercial guidance can structure the conversation, but it cannot replace that assessment.

Fabrication also influences the result: welds, heat treatments and surface finishes can modify the corrosion behaviour of a correctly chosen material. That is why material specifications usually come with fabrication and testing requirements that must be defined in the project, not assumed.

Names such as Hastelloy or Inconel are trade marks designating families of nickel alloys from their respective owners. They are cited here as recognisable examples of material families; mentioning them does not imply any commercial relationship with their owners, guaranteed availability or suitability for a specific service.

Some decisions seem harmless in a first conversation and end up conditioning the whole project: fixing a material because «it has always been used», ruling out cladding out of unfamiliarity or assuming that an alloy solves any aggressive medium. None of these decisions should be taken without assessment by materials and corrosion specialists, with the service data on the table and the applicable standards verified in their current edition.

The most productive attitude at the purchasing stage is an open range: describe the service well, provide the available previous experience and ask the manufacturer to justify the proposed material with the criteria used. That justification, rather than the material's designation, is what allows quotations to be compared soundly and the decision to be documented before third parties.

• Generic nature of the fluid: acidity, chlorides, sulphides, abrasion, without disclosing confidential formulations.
• Preliminary temperature and pressure conditions and their expected variation.
• Previous experience with materials in similar services, if any, as a reference rather than a specification.
• Cleaning, inspection and expected service-life requirements that may condition the selection.
• Who will carry out the project's materials and corrosion assessment.