Corrosion characteristics of fatty acids in water solutions

Abstract

The main investigation consisted of determining corrosion rates of mild steel (A lSI 1 020) exposed to de-aerated stagnant solutions of fatty acids in water at different acid numbers and temperatures, with the aid of mass loss tests. The temperature range investigated varied from 25°C to 250°C. Results showed that corrosion rates initially increased with temperature and at certain elevated temperatures thick multi-laminated corrosion product films formed on the specimen surfaces which .led to decreases in corrosion rates with further increases in temperature. Corrosion product films influenced the control mechanism of the corrosion process and it was seen that the characteristics of these films were dependent on the temperature as well as fatty acid type. The acid number range investigated was from 0 to 25 mg KOH/g solution. Corrosion rates increased with an increase in acid number irrespective of the control mechanism. Results indicated that the corrosivity of acid solutions decreased with an increase in chain length of the organic acid molecule, when no corrosion product films were present. The presence of corrosion product films rendered complex relationships. Electrochemical tests were done on specific fatty acid systems at 25 °C. Corrosion rates evaluated from electrochemical observations correlated well with mass loss results and it was concluded that the corrosion mechanism was electrochemical. Supplementary investigations were conducted on specific fatty acid systems and included the following: Turbulence did not influence the corrosion process. This is typical for an activation-controlled system where no corrosion product films are present on the specimen surfaces. Mixing of acids showed a corrosion rate of approximately an average of the two individual corrosion rates of the pure systems. Monitoring studies showed that results obtained with the Corrator correlated better with mass loss and electrochemical corrosion rates than Corrosometer results. Corrosion rates increased with an increase in dissolved oxygen content level of the solution and with additions of chloride, but sulphur additions rendered lower corrosion rates. A water phase process stream received from SASOL showed that the corrosion rate increased with temperature and levelled out towards 200°C The corrosion characteristics of the received stream can be simulated by acetic acid with the same acid number.

Die hoofondersoek het bestaan· uit die bepaling van korrosietempo's van laekoolstofstaal ( A lSI 1020 ) wat blootgestel word a an ontlugte stagnante oplossings van organiese sure in water by verskillende temperature en suurgetalle. Die korrosietempo's is bepaal deur gebruik te maak van massaverliestoetse. Die temperatuurgebied wat ondersoek is, was van 25 °C tot 250°C. Dit is waargeneem dat die korrosietempo aanvanklik toeneem met 'n toename in temperatuur en daarna is 'n afname in die korrosietempo waargeneem. Die afname word toegeskryf a an 'n dik multi-laminere-korrosieprodukfilm wat op die monsteroppervlak gevorm het. Hier is waargeneem dat die beheermeganisme be"invloed word deur die teenwoordigheid van korrosieprodukfilms terwyl eienskappe van die films afhanklik was van die tipe suur, asook die temperatuur waarby eksperimente gedoen is. Die suurgetalgebied wat ondersoek is, was vanaf 0 tot 25 mg KOH/g oplossing. Korrosietempo's het toegeneem met 'n toename in suurgetal ongeag die beheermeganisme wat bestaan het. Die korrosiwiteit van die suuroplossings het afgeneem met 'n toename in organiese molekuullengte, waar geen korrosieprodukfilm teenwoordig was nie. Die teenwoordigheid van korrosieprodukfilms het komplekse korrosietempo-tipe suur verhoudings by konstante temperature opgelewer. Elektrochemiese toetse is gedoen op sekere sistema by 'n temperatuur van 25 °C. Korrosietempo's verkry deur gebruik te maak van elektrochemiese toetsmetodes vergelyk baie gunstig met massaverliestoetsresultate. Die gevolgtrekking wat dus gemaak is, was dat 'n elektrochemiese korrosiemeganisme bestaan. Newe-ondersoeke is gedoen op sekere .organiese suursisteme en het die volgende ingesluit: Turbulensie het geen effek op die korrosieproses gehad nie. Dit is tipies wat verwag word van 'n aktiveringsbeheerde sisteem waar geen korrosieprodukfilms teenwoordig is op die monsteroppervlak nie. Mengsels van die sure het getoon dat die korrosietempo's 'n gemiddelde waarde van die individuele korrosietempo's van die suiwer suursisteme oplewer. In die moniteringstudies het die Corrator resultate beter gekorreleer met massaverlies en elektrochemiese toetsresultate as Corrosometer resultate. Die korrosietempo het verhoog met 'n toename in opgeloste suurstofinhoud van die oplossing asook met die byvoeging van chloriede, terwyl 'n afname in korrosietempo waargeneem is met die byvoeging van swael. 'n SASOL waterfase produkstroom het getoon dat die korrosietempo toeneem soos die temperatuur toeneem tot ongeveer 200°C waarna 'n afplatting in korrosietempo waargeneem is. Die korrosiekarakteristiek van die SASOL waterfase produkstroom word redelik goed verteenwoordig deur 'n asynsuur sisteem met dieselfde suurgetal.