Plant

Tackling corrosion in urea processes

An interview with Mr. Kirk Ofei from Stamicarbon, who enjoys working on challenging and interesting projects that test his intelligence and qualifications and increase his knowledge.
 
As Mr. Ofei is a Materials & Corrosion Specialist at Stamicarbon, I was very keen to hear his thoughts. Mr. Ofei earned his MSc degree in Materials Science at the University of Aveiro, Portugal and Aalborg University, Denmark.

Mr. Ofei joined Stamicarbon in June 2013 as Materials & Corrosion Specialist within the Inspection & Materials Engineering Department. In this role he is responsible for the materials & corrosion expertise. In a blog posted in 2015 on the Stamicarbon website he said “You have to be imaginative; of course you need your analytical skills, but you also need to have the ability to imagine solutions outside what is presented to you. Problem solving has become a part of who I am.”

I've started by asking Mr. Ofei about his function and the responsibilities this entails. “As the Materials & Corrosion Specialist for Stamicarbon, my function is split into two main roles. The first is working on innovation projects, where I manage R&D on materials and corrosion. For example I was involved in the development of Safurex® Star, a superduplex stainless steel developed in collaboration with Sandvik for use in the highly corrosive urea synthesis environment. Another aspect of my role is to develop, maintain and record knowledge on materials & corrosion.

I also act as the contact person to relevant laboratories, institutes, equipment manufacturers and suppliers with whom we collaborate.” “My second main role is to offer technical support with respect to materials and corrosion to urea plants worldwide. This includes troubleshooting corrosion related issues on and off-site. I also execute inspections in urea plants and coordinate the activities on site.

Finally, I give trainings to customers and presentations at external symposia within the domain of materials & corrosion.” When asked what corrosive processes he is involved in, Mr. Ofei replied: “We work at preventing the following corrosive processes: general/uniform corrosion, condensation corrosion, strain induced intergranular corrosion (SIIC), pitting and crevice corrosion, flow accelerated corrosion, corrosion under insulation, chloride stress corrosion cracking and nitrate stress corrosion cracking.”

Material selection

“In terms of the corrosion resistant alloys (CRA’s) that we use, these are employed in the high pressure synthesis section of Stamicarbon urea plants where process conditions are acidic and temperatures range from approximately 180 to 205 °C. Such process conditions call for mechanically sound materials with excellent corrosion resistant properties, but which are also relatively easy to fabricate and cost effective.”

When questioned as to which grades of stainless steels and/or duplexes Mr. Ofei commonly works with, he replied: “As a standard, only Safurex® (UNS S32906), a proprietary superduplex stainless steel, is employed as the material of construction for all wetted parts in the synthesis section. For less corrosive applications, lean and medium duplexes (X2CrNiMoN22-5-3 and X2CrNiMoCuN25-6-3) are used. We also use the X2CrNiMo25-22-2, 304 and 316 grades of austenitic stainless steels when the service environment requires it.”

“After more than 20 years of employing the Safurex® super duplex material, most of the corrosion phenomena associated with high pressure synthesis sections of urea plants traditionally constructed from austenitic stainless steels (e.g. 316 UG and X1CrNiMoN25-22-2) have been eliminated. Corrosion is hardly measurable on liner plates of high pressure vessels.”

Very few other CRAs are used at Stamicarbon explained Mr. Ofei. “Although titanium is a well-known corrosion resistant material, it is hardly ever specified as a material of construction in our plants, apart from being used to manufacture some components in pressure transmitters. Aside from the issues of cost and weldability, it is susceptible to hydrogen embrittlement in oxygen free ammonium carbamate and erosion corrosion. Nickel alloys are not suitable for use in urea plants due to the likely formation of nickel-ammonia complexes which reduce their corrosion resistance.”

“We use lean duplexes in ammonium carbamate environments for temperatures of approximately 104°C, and our experience with them with respect to corrosion resistance is positive. They perform well under such service conditions,” Mr. Ofei explains.

Facing corrosion issues

When questioned about what corrosion or welding issues he encounters, Mr. Ofei he replied: “We recently dealt with unexpected failures of high pressure gas lines made from 316 UG material. The failure led to leaks in these lines. The leaks were all located near weld connections in pipelines which were 30 years or older. All the lines were traced and were included in a regular inspection plan, but the failures were not detected. Rigorous examination of samples from the failed pipes revealed a strain induced intergranular corrosion phenomenon initiating from corrosion products under condensation spots.”

Mr. Ofei continues explaining how he dealt with the issues: “The inspection plans for pipelines (based on RBI philosophy) made from the same material under the same service conditions have been updated to include ultrasonic scans at likely locations. We are also advising that insulation and tracing be well maintained and operative at all times. If such pipelines are to be replaced, we recommend using super duplex grades which are immune to this phenomenon (in HP gas lines in urea plants). ” Out of interest I asked Mr. Ofei what the most common corrosion failure he has come across: “This has to be uniform/ general corrosion, however, with proper materials selection and good operating practices the corrosion rates for this phenomenon can be reduced to almost negligible values.”

CRA challenges

Asked about the biggest challenge with the CRA’s right now Mr. Ofei replies: “There isn’t a shortage of creative ideas with which to make new things, usually the main drawback to these ideas are the materials to bring them to reality in a cost effective way. The current challenge is making cost effective CRA’s for demanding service environments.”

In terms of procurement, Mr. Ofei said: “In Stamicarbon we have a procurement department in place to handle all matters related to purchasing. However for Safurex® which is a proprietary material, we have a single source supplier.” “In my role, I am usually not in direct contact with CRA suppliers, however, I have learned from equipment manufacturers that lead times for delivery are usually long,” explains Mr. Ofei.

“This means projects have to be planned well ahead of time and emergency replacements can be difficult to arrange. However, for the HP urea equipment the lead time of carbon steel forgings (tubesheet, hemihead) is longer compared to Safurex®; Safurex® is almost never on the critical path!” “Looking to the future, I would like to see further optimisation of lean duplexes to improve corrosion resistance through clever heat treatment regimes. This is because, there is a limit to how far corrosion resistance can be improved by alloying, therefore investigating how corrosion resistance can be improved by strengthening the microstructure through heat treatment is important.”

 

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