was analysed by X-ray photoelectron spectroscopy (XPS) (see Figure 2 ). The peak around ~131 eV binding energy for the elemental phosphorus shows the formation of a phosphorus-based protective layer on the corrosion inhibi- tor-treated metal surface, leading to a 74% corrosion inhi - bition compared with the untreated system. Further surface characterisation of phosphorus-based corrosion inhibitor-treated metal (tested in the HVGO medium containing TAN and 1 wt% sulphur; temperature ~ 325ºC, P5 metallurgy) was studied by time-of-flight sec - ondary ion mass spectrometry (ToF-SIMS) and cross-sec - tion analysis by scanning transmission electron microscopy (STEM). Figure 3 shows the presence of phosphorus and sul- phur-based protective film/scale over the metal surface. The ToF-SIMS depth profile studies on the metal surface showed ~150 nm of phosphorus-based passivation layer on the metal surface, and sulphur was detected at the phos- phorus and iron oxide interface. The STEM analysis shows a ~100 nm protective layer on the corrosion inhibitor- treated metal surface. Laboratory investigation of naphthenic acid corrosion The high-temperature naphthenic acid corrosion processes in the field are very complex. They can be influenced by multiple operational and fluid compositional factors such as temperature, pressure, shear stress, naphthenic acid spe- cies and their concentration, as well as sulphur species and their concentration, in addition to unit metallurgy. To bet- ter understand the significance and interaction of all these factors, the performance of multiple corrosion inhibitors was systematically evaluated through extensive laboratory experiments designed to simulate various field conditions. The corrosion evaluation was conducted in accordance with the ASTM G184 standard method, which simulates pipeline corrosion conditions.3 The rotating cage autoclave (RCA) equipment used was fabricated in compliance with the specifications given in ASTM G184. RCA is considered a top-ranked methodology for corrosion inhibitor evalua- tion and qualification for pipeline applications. Based on the pipeline design and operating conditions, the laboratory testing protocol is set to evaluate similar field conditions for high shear stress, temperature, and pressure.
Corrosion rate increases
Corrosion resistance increases
317SS
316SS
4 10 SS
12Cr
9Cr
5Cr
CSteel
Most resistant
Least resistant
Enhancement of metallurgical specifications (see Figure 1 ) can be effective in improving corrosion resistance but may not always be feasible as it involves a significant invest - ment of capital and time. Another approach to mitigating risks associated with high- acid crude processing is the utilisation of high-temperature corrosion inhibitors (CI) and the establishment of optimised chemical dosage protocols. Furthermore, the selection and implementation of appropriate inhibitors, coupled with stra- tegic application methodologies, enables refineries to pro - cess feedstock containing elevated acid concentrations. This capability enhances operational flexibility and potentially yields improved financial performance through the process - ing of more economically advantageous feedstocks. High-acid crudes corrosion inhibition by chemical treatment Phosphate or phosphonate esters, sulphur (S) compounds, and combinations of sulphur and phosphorus (P)-based chemistries tend to inhibit corrosion at high temperatures and are commonly used as naphthenic acid corrosion inhib- itors. The corrosion inhibitor is expected to form a protective layer of insoluble phosphates and iron sulphide, protecting against the formation of hydrocarbon-soluble and corrosive iron naphthenate.2 An analysis of a metal surface to which an inhibitor treatment is applied, demonstrating corrosion mitigation, is discussed in the following section. Advanced analytical surface characterisation A heavy vacuum gasoil (HVGO) sample (TAN~1.35 and S~1.58 wt%) from a US refinery was tested for corrosion studies (325°C; 3 hr, shear stress ~30 Pa) on carbon steel metallurgy with and without phosphorus-based corro- sion inhibitor. Post-experiment, the surface of the metal Figure 1 Naphthenic acid corrosion mitigation by metal - lurgy upgrade
25 , 000
Test uid - US Re nery HVGO
Corrosion rate (mpy)
20 , 000
No treatment
22.2
Phosphorous - based Cl
5.8
15 , 000
P - based Cl No treatment
10 , 000
5 , 000
0
0
200
400
600
800
Binding energy (eV)
Figure 2 XPS Survey scan of metal samples from untreated and phosphorus-based corrosion inhibitor-treated system
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PTQ Q2 2025
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