Corrosion Testing for Risk Reduction in Chemical Process Development - 1
Corrosion Testing for Risk Reduction in Chemical Process Development - 1
Safe and reliable introduction of new process technologies
This work explores some of the key factors to consider in design and implementation of corrosion testing at a laboratory scale for the development of new chemical technologies in order that process technology scale-up risks, not least those of safety, can be minimised. This is to ensure safe and reliable introduction of new process technologies, while also pursuing the minimum capital cost of often expensive plant materials of construction (MoC). Laboratory-based corrosion testing should never be used exclusively to replace inspection and monitoring of corrosion in operating process plants, as real-world conditions are rarely possible to be wholly replicated in the laboratory. However, testing as initial screening, or to provide deeper mechanistic insights is often an essential part of the development and design of first-of-a-kind process technologies. Several methodologies to assess corrosion under highly aggressive conditions have been developed and applied in the development of new chemical processes and are demonstrated in two case studies outlined in this article. This work focuses on testing of materials in contact with corrosive fluids.
The selection process for MoC for new chemical technologies is often challenging (1). The corrosion or materials degradation risks can be severe as demonstrated in incidents such as the NDK Crystal Inc explosion in the USA in 2009 (2), or at smaller scale by the examples in Figure 1. Corrosion performance must be understood to avoid immediate or prolonged plant failure and ensure lifetime plant integrity (3, 4). It is the responsibility of the engineers that design the equipment and operational controls to consider what could go wrong and devise means to mitigate potential risks. This article will set out to highlight the wide range of considerations taken by those selecting MoC in the chemical process industries, but also provide some brief case studies of how testing can be successful to enhance equipment life and ensure safe operation.
Corrosion in chemical processes is the detrimental reaction of the MoC containing the process fluids with the process conditions themselves resulting in degradation of the material of construction, and often resulting in failure, repairs or other detrimental or costly outcomes. Corrosion often manifests as either uniform or localised.
Uniform, or general, corrosion refers to mechanisms that result in homogeneous thickness loss. It attacks all exposed metal surfaces equally. High uniform corrosion rates can lead to wall-thinning and insufficient wall thicknesses to maintain structural integrity, or lead to malfunctioning equipment.
Localised corrosion preferentially corrodes material at small, possibly difficult-to-detect regions, via mechanisms such as crevice, intergranular or pitting corrosion, as seen in Figure 2 and Case Study 2. While there is a number of mechanisms, there are often commonalities, such as corrosion promotion under stagnant flow conditions or a build-up of corrosive ion species in isolated locations. In addition, a build-up of metal ions from the corrosion of one localised region can influence corrosion elsewhere in a chemical process plant. The formation of corrosion pits and crevices can cause undetected, rapid and unpredictable deterioration and ultimately lead to equipment failure due to breaches in the pressure boundary, posing significant risk to operators and the surrounding environment.
When designing chemical plants, a flowsheet for the process needs to be initially defined but typically comprises of sections as illustrated in Figure 3. Due to the different conditions of operation required for each of these units, they need to be evaluated individually for their potential corrosivity; these are often called corrosion loops in risk-based inspection analysis (American Petroleum Institute Recommended Practice (API RP 581) (5)).
In the development of a novel chemical process the evaluation of corrosion performance and materials selection becomes increasingly important, and experimental work is often required to inform these choices.
2. Challenges Selecting Materials of Construction for Corrosive Processes
The open literature and operator experience under similar conditions can often be a valuable source of information to predict the behaviour of materials in various plant environments, aiding the materials selection process. However, these aids are less helpful when developing novel chemical processes that do not conform to previously documented conditions, those presented in literature, or where it is suspected that seemingly small variations in the conditions from that of the reference might make a meaningful difference in material performance. Where limited reference work exists, there is a need to design custom corrosion testing programmes which enable the evaluation of candidate materials under representative test conditions.
3. Challenges Using Accelerated Corrosion Testing Data
When generating corrosion test data, it is critical to consider the full range of plant conditions, which in many cases includes those outside of where the plant is designed to normally operate.
3.1 Transient Conditions
These alternate conditions can be for several transient reasons, such as upsets, start up and shutdowns, catalyst regeneration, cleaning, steam out or exothermic runaway reactions. Additionally, more longer-term changes such as adjustments due to catalyst ageing, optimisations to plant performance due to equipment fouling, changes in feedstock, product purity targets or a myriad of other reasons also need to be considered. A thorough understanding of the existence and likelihood of these variations, but also their impact on critical materials damage mechanisms, is required to ensure materials testing programmes incorporate these variations. This is of particular importance where materials may rapidly deteriorate outside of the planned operating conditions but within the plant design conditions, where it is customary to have sufficient design margin to allow for short-term transients. However, an operator may not be aware of the damage occurring to the plant if they consider safe operation to be within the plant design limits. Where rapid materials damage is identified within the possible design conditions of the plant, this materials test data can be used to set safe integrity operating windows following industry best practice such as set out in API RP 584, ‘Integrity Operating Windows’ (6).
3.2 Material Form or Condition
Materials and corrosion engineers understand that materials performance can be strongly influenced by product form (see Case Study 1), quality, processing parameters and modifications. For instance, a cast variant of an alloy may corrode significantly faster than that of its wrought counterpart. It is therefore of significant importance for those designing corrosion tests for MoC of critical process equipment to understand these influences on performance, but also the design and fabrication of such equipment in the real world. Materials that would need to be considered are not only the main pressure boundaries, typically grades of steel, but can be more exotic alloys, as well as internal cladding and lining materials, O-rings, valve seats, packing, trays and demisters. Many of these items of construction are of fundamentally different materials to those of the pressure boundary but can all be altered in their corrosion performance through heat treatment or forming parameters, welding materials and practices, surface contamination, pickling and passivation or inherent internal stresses. Materials and corrosion engineers need to work closely with other engineering disciplines to tease out early in the development of new process technologies what the possible final equipment design may be, as this can also influence the possible materials that can be utilised. For instance, plate and frame heat exchangers have a more limited choice of alloys available than that of shell and tube.
Accelerated testing presents limitations in the evaluation of long-term damage mechanisms, such as high-temperature hydrogen attack. An understanding and awareness of the characteristics and prevalence of such mechanisms is important to ensure sound decision-making at any initial material screening stage.
To complement corrosion testing, monitoring once a plant is online can also be employed. Corrosion monitoring, for example in the form of electrical resistance probes, ultrasonic wall thickness monitoring or retrievable coupons, can be used to mitigate corrosion risks whilst an operating plant is in service. However, a thorough understanding of the potential for corrosion is highly desirable as part of the development of an optimised corrosion monitoring programme.
4. Design of a Corrosion Testing Programme
Published standard practices and guides to materials testing such as those issued by the American Society for Testing and Materials (ASTM), for example ASTM G1-03, ASTM G31-21, ASTM G48-11, ASTM G111-21 (7–10), can be used in the design of corrosion testing programmes and ensure the development of robust protocols. The use of standard operating procedures, structured reporting and data collection systems strives for testing that demonstrates repeatability and reproducibility.
When planning a corrosion testing campaign, it is important to plan in detail the range of materials, compositions, processing parameters and forms, as these can be critical in identifying corrosion risks. Typical examples of coupons used in a corrosion test can be found in Figure 4, which illustrates that some coupons have been welded, with the knowledge that welds would be included in the final equipment item.
4.1 Breadth of Test Environments Covered
The range of temperatures and pressures experienced throughout a chemical process plant result in a range of equipment being required to fulfil the needs of a testing programme. Several types of equipment can be applied and modified to achieve the best possible environment at any condition, as demonstrated by Figure 5 which shows an example of some of the range of wet corrosion testing capabilities available in a hypothetical corrosion testing laboratory.
Glassware setups are frequently utilised as they can achieve temperatures over a wide range but are limited by their use at ambient pressure due to evaporation of test fluids and their inability to hold significant pressure. In most test conditions, glass is inert to the test fluids allowing testing of extreme corrosivity and minimising influence of contamination both on the test and analytical measurements. Where moderate pressure conditions are required glass pressure vials may be used. However, these do not easily allow for active stirring of the test fluids or simple means by which to control the chemistry of the gases in the vapour space or those saturated in the test fluid, so need to be applied only where appropriate. One of the advantages of glass is the ability to visually monitor the corrosion test throughout, so signs of corrosion can be used to end a test early and move onto other candidate materials.
Autoclaves are the workhorse of high-pressure corrosion testing of materials and can be set up in many configurations to allow for a vast range of test conditions and online analysis. Where more advanced dynamic test conditions at higher pressures are required pilot plants represent the best opportunity for the most representative test conditions, but at the greatest expense and investment.
4.2 Test Coupons
Sourcing and preparation of test materials is fundamental to a successful corrosion test. Material provenance must be established to ensure the materials to be tested are from a representative source, suitable for the chemistry and in good condition. Acquiring test samples from suitable sources can be challenging and often involves building strong relationships with a range of global suppliers and producers.
As part of the initial planning of the corrosion test it must be decided what the likely product forms are and fabrication practices that may be used to construct the final scale plant. These materials and forms must then be assessed by a corrosion engineer to establish if such material forms can meaningfully influence the corrosion performance of the materials. If there is a possibility of form influencing corrosion performance, then coupons must be sourced or conditioned in such a way to represent this in the corrosion testing. For instance, this may involve use of welded coupons (see Case Study 2), or subjecting the coupons to a thermal heat treatment, or pickling step before use in the testing programme.
The selected coupon material must also be cut to the required size for use in the test; this must be of sufficient size to provide sufficient test area, but also fit within the equipment. The processes used for cutting the coupons must not influence the performance of the coupon materials, which generally means that thermal processes should be avoided or compensated for. For example, laser cutting or heavy machining without coolant.
Following coupon preparation, samples should be cleaned such that they are free from the influence of any surface contamination. In the case of many metallic alloys embedded iron can act to enhance corrosion of the coupon, so this must be avoided in their preparation.
In special circumstances, some coupons need to be shaped or conditioned such that they can simulate factors that might exacerbate corrosion in service, for instance coupons can be stressed to facilitate stress corrosion cracking (see Figure 6).
4.3 Test Methods
This section outlines test methods than can be used to ensure representative corrosion testing.
Batch testing is typically conducted in either glassware or simple autoclaves. The glassware used is a round bottomed flask or similar which can be stirred with a coated magnetic flea or a top-down mixer. Flask lids can be sealed to allow control of the apparatus vapour space, with multiple ports for stirrer access, thermowells, pH monitoring or liquid and gas input and output ports.
Where conditions slightly above atmospheric pressure is required to maintain volatile components in the liquid phase, sealed glass pressure vials can be used alongside appropriate vapour pressure calculations and safety precautions. Multiple vials can be placed together in a hot oil bath to allow for multiple concurrent tests at a single temperature. These vials are ideal for screening many alloys or process conditions rapidly to home in on a material, family of materials or fluid chemistry for more detailed study.
Autoclaves come in a wide variety of sizes, materials, capacities, temperature and pressure ratings and are frequently used for high pressure and temperature corrosion testing. As autoclaves are exclusively manufactured from metallic alloys it is possible for minor corrosion of the autoclave to influence a corrosion test. Care and consideration must be taken to ensure coupons are held appropriately to avoid electrical contact with the autoclave and other coupons. The use of polymeric liners is one method to isolate the metallic pressure boundary of the autoclave from the test fluid and coupons.
Challenges to produce representative corrosion test environments for the required duration in batch style tests have been outlined. In response to this, several test methodologies to compensate have been developed, which include the following:
Periodic refreshment of test fluids: simple to implement, typically targeting a minimum corrosivity resulting in on average a more corrosive test than the target minimum, or targeting an average corrosivity cycling above and below the minimum expected on plant which is challenging to gather accurate corrosion rate measurements from (see Case Study 1)
Circulating test fluids with metered fluid refreshment: more complex to set up, but allowing fine control of the test environment
Once through system: where the test fluid is only passed over the test coupons once, or with a short residence time over the coupons. Only suitable for very volatile chemistries or low-test volumes (see Case Study 2 for an example of this).
Ensuring not only the test liquids are representative of the process being modelled, but also the vapour space chemistry and dissolved gases too is often vitally important. Dissolved oxygen is typically of great importance in corrosivity, and in chemical processes where dissolved gases are part of the process chemistry (for example, hydrogenation), this can also heavily influence a test’s representative corrosivity. It is therefore vital to ensure we can control the gases present in the test, be that through gas blanketing with inert gases (such as argon or nitrogen) or through sparging.
Within a test environment we are also able to simulate factors known to influence the corrosion of materials in service, some example methods widely employed are:
Crevice corrosion testing: where a crevice of defined tightness is simulated to produce a corrosive environment local to the material surface (an example of a standardised guide is ASTM G78-15) (see Case Study 2 for application of this methodology) (12)
Galvanic corrosion testing: where two conductive materials (typically metal alloys) are in electrical contact in the same corrosive environment and their comparative electrochemical potential in that environment can drive accelerated corrosion of one member of the galvanic couple (standardised guide ASTM G71-81) (13)
Stress corrosion testing: where either stressed material in the presence of a corrosive environment can fail due to cracking. Often the environment is considered mild due to lack of obvious corrosion of the surface, however local to applied stress, cracks can be propagating through the material. Stresses can be from static or dynamic loads on the material or can be residual stresses from the fabrication such as heat treatment, welding or forming. Multiple test standards exist for different environments, applications and alloys. ASTM G30 sets out methods for preparing certain types of stressed specimens (see Figure 6) (11)
Liquid-vapour interface testing: where not only are corrosion coupons able to be placed in either the liquid or vapour phase of a process, but also at the interface of both fluids, as unique corrosive conditions are able to exist due to differences in chemistry at the interface
Corrosion testing in heat transfer conditions: where a coupon is heated or cooled from one side to produce a localised heat flux, as it is possible for corrosion performance to vary under such conditions
Mini-plant and pilot plant: often considered the next best thing to measurement of corrosion in a working plant, coupons placed into pilot plants or smaller scale mini-plants can be employed (see Case Study 1 for one application of this). During the development of new chemical processes pilot plants are built and can incorporate corrosion testing coupons or sacrificial components that can be destructively tested following some period of operation.
5. Analytical Methods
Following a corrosion test there are several analytical methods that can be employed to assess the state of the materials tested. These broadly fall into two categories, physical and chemical.
5.1.1 Visual Assessment
Simple inspection by eye can be used to analyse material coupons identifying the presence of obvious features associated with corrosion, some being topographical, such as roughness, pitting, etching, cracking or changes in colour (for example, rusting).
For assessment of less obvious corrosion damage, optical, scanning electron or even atomic force microscopy (AFM) can be used to identify damage on the microstructural level. This can be done simply on the surface following careful removal of corrosion products where they remain, either mechanically or chemically.
Damage associated with certain microstructural features or metallurgical phases can be more readily identified through metallographic preparation of a cross-section of the surface of the material (see Figure 7). This is particularly powerful when combined with etching to identify microstructural features or mapping of the surface chemistry with more advanced techniques such as energy dispersive X-ray spectroscopy (EDS). Some other corrosion mechanisms, such as selective phase attack, pitting or stress corrosion cracking, can be less apparent without cross-sectioning.
5.1.2 Mass Change
A key parameter for metallic coupons is the calculation of corrosion rate from mass loss. Using dimensional and weight measurements taken pre- and post-test, corrosion rates can be calculated. However, standard corrosion rate calculations assume uniform corrosion; this is a limitation of this parameter as it can often manifest as localised corrosion such as pitting or crevice corrosion, as was the observation made in Case Study 2. Equally a material can gain weight where the corrosion products are still adherent or part of the final coupon.
5.1.3 Mechanical Properties
In specific material/environment combinations corrosion can manifest as a change in material performance. Where such conditions are of concern simple screening can be performed through mechanical testing, for instance, hardness, bend test and tensile testing.
Often corrosion products, either remaining on the surface of the sample, or in the test fluid can manifest with obvious colour changes. This can be a first clue as to corrosion occurring, but care must be taken to confirm that such colour changes are due to corrosion.
Chemical clues can be powerful tools in identifying the presence of corrosion and elucidating by which mechanism corrosion is proceeding.
Chemical analysis of the following is common as part of a corrosion test programme:
Establish baseline chemistry of material to be tested for corrosion performance
Surfaces or microstructures that form following corrosion on or within the sample coupon
Test fluids before, during and after exposure to the test material
Suspected corrosion products.
Chemical analysis prior to testing of the coupons is vital to ensure representative or worst-case composition and metallurgical condition. Following exposure where other techniques might struggle to identify detrimental changes in the test material following inevitably short test durations and chemical changes are suspected, these can be used to identify corrosion, for instance, dealloying of the surface, or hydride formation with the sample interior. Chemical mapping, such as EDS mapping, can be a particularly powerful tool when combined with knowledge of the test coupon microstructure.
Chemical analysis of the test fluids to ensure the test maintains a representative environment is vital, for instance maintaining the correct acidity can be measured either through pH or total acid number (ASTM D664-09) (14). This was used in both case studies presented. Methods such as inductively coupled plasma-optical emission spectroscopy/mass spectroscopy (ICP-OES/MS) can provide indication of corrosion occurring due to presence of signature elements of the material being tested within the test environment. Such measurement can be used to powerful effect when monitoring for corrosion as part of an online test monitoring system. There is a wide range of alternative techniques available to analyse test fluids, ICP-OES being one of them which was used in Case Study 2. The Johnson Matthey analytical laboratories in Sonning Common, UK, provide access to powerful chemical and imaging techniques.
6. Case Studies
6.1 Case Study 1: Extreme Environment Testing
For one of the methodologies developed, metal corrosion coupons were evaluated under high temperature and pressure (200°C and 100 barg) with a highly corrosive process fluid containing a mixture of organic and mineral acids. This test work required the use of a continuously operated mini plant with a custom designed fixed plug flow reactor lined with tantalum, a material inert to the highly corrosive conditions at the required temperature and pressure. These conditions are required to facilitate the first stage reaction for a preparatory commodity chemical process. In the past, screening tests had been performed in a batch autoclave. However, due to highly reactive components the process fluid was found to be unstable under the test conditions. A continuous mode of operation was developed (flow loop) to ensure the corrosion coupons were exposed to stable and representative process conditions for the entirety of the test, matching that of the final plant design.
Process fluid was pumped into the reactor using a piston pump and collected upon exit, ready to be recycled back again as feed during the test. If the feed composition changed considerably over a longer period (as per change in the total acidity), it could then be replaced with a fresh batch of process fluid. In this test a custom polytetrafluoroethylene (PTFE) coupon holder was also designed to allow for testing of multiple coupons in a single run while also allowing suitable flow of the test fluid over the coupons and isolating them from both the vessel wall and each other.
As shown in Figure 8, it was found in these tests that very small changes in the reactor temperature proved to have a large effect on the corrosion rate, which was critical in the identification of necessary operating window limits for a particular alloy for this flow loop.
6.2 Case Study 2: Low Acid and Water Fraction Crevice Corrosion Testing
In another methodology developed, corrosion tests were conducted in a glassware setup (see Figure 9) with a once-through configuration. A residence time of 30 min was required to achieve a representative liquid composition seen in the reboiler of a distillation process where there are both low levels of organic acid and water present (≤0.5%) at high temperatures (150–200°C). A peristaltic pump was utilised to pump the feed into the reactor, in addition to a small continuous flow of low-pressure nitrogen gas and a PTFE dip leg for level control, ensuring appropriate control of the vapour space chemistry and in removing liquid product. Due to highly corrosive conditions, special custom-made PTFE thermocouples for temperature measurement were specifically acquired for this test work and reliable heating was achieved through the use of both a heating mantle and heating tape. Frequent analysis of the liquid product by ICP-OES was conducted to assess for any metal leaching during the tests.
In one of the early tests conducted, crevice corrosion was identified by the corrosion engineer on some of the metal coupons that otherwise showed acceptable rates of corrosion. As a result of these observations, the use of castellated ceramic crevice washers was subsequently implemented in future tests to evaluate crevice corrosion more consistently, as shown in Figure 9. Using a defined clamping pressure this method ensures that the inherent uncertainty associated with crevice formation is accounted for and the possibility of crevice corrosion conditions is more readily created in the tests. These tests demonstrated the possibility of enhanced corrosion in crevices under these plant conditions for the selected alloy and was used to inform plant materials selection to a more resistant alloy and reduce the risk of early life failures.
This paper has aimed to provide an overview of considerations and techniques to test, measure and understand corrosion that may manifest in novel chemical process conditions created as part of new chemical process development. Such testing can be used to select MoC more accurately to contain these processes, thus reducing corrosion risks but also allowing optimisation of the material selection and process conditions for safety, reliability and commercial gains.
It may be evident to the reader that there are many considerations that need to be made by the designer of such testing requiring a broad spectrum of capabilities, including detailed knowledge of corrosion mechanisms, laboratory chemistry expertise, process chemistry expertise, experience of real world operating practice, process equipment design and fabrication expertise, materials chemistry and microstructures expertise, access to advanced materials interrogation techniques and experience, and many others. Corrosion testing of this kind therefore requires a strong multidisciplinary team with access to the required tools and capabilities to perform and interpret such tests.
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Jarle Holt received his PhD from the Norwegian University of Science and Technology, Trondheim, Norway in 2006. He spent time as a postdoctoral researcher at Delft University of Technology, The Netherlands and the University of Leeds, UK, before joining Johnson Matthey in 2011 as a Chemist. His current position is as Principal Chemist, working in the licensing technology business. The main research interests are around developing homogeneous and heterogeneous catalytic processes.
Katie Atkins received her MEng in Materials Science and Engineering from The University of Sheffield, UK, in 2018. She joined Johnson Matthey in the same year and her current role is as a Materials and Corrosion Engineer in the licensing technology business.
Stephen Shapcott is Senior Principal Materials and Corrosion Engineer for Johnson Matthey in the licensing technology business. Stephen holds a BEng in Materials Science and Engineering, as well as an MRes in Corrosion Science and Engineering from The University of Birmingham, UK. Stephen spent four years as a Corrosion Researcher with BAE Systems, followed by two years as a Materials Engineer working in offshore oil and gas before joining Johnson Matthey in 2015. Stephen is a Chartered Engineer and Fellow of the Institute of Materials, Minerals and Mining.