Abstracts Category : Other

Inhibition of mild steel corrosion in cooling systems bylow- and non-toxic corrosion inhibitors

by Mohamed Ali Ahmed

AbstractThe aim of the research in this thesis wasto study how environmentally friendly corrosion inhibitors forcooling water systems might be developed and used. Firstly, reducedtoxicity inorganic corrosion inhibitors (i.e. nitrite/molybdate)were considered. Secondly, non-toxic inhibitors based on mono anddi-basic salts of carboxylic acids were studied systematically as afunction of carbon chain length. For nitrite inhibitor alone, aconcentration of 7 mM NaNO2 was effective to inhibit carbon steelin chloride media of 10 mM NaCl, while 10 mM nitrite was needed insulphate media of 3.66 mM Na2SO4. However, it was found possible tosignificantly reduce the concentration of nitrite by addingmolybdate in synergy. This was attributed to the nitritepassivation combined with ferrous molybdate salt film pore pluggingthus promoting a continuous and protective film on the materialwithin these media. Thus, in pH 6-10 an inhibition efficiency of97% was recorded with a mixture of 3 mM nitrite/2 mM molybdate inboth chloride and sulphate media and at 25C and 60C. However asthe solution pH decreased below pH 4 the inhibition efficiencydecreased to about 47%.In the second part of the study, the use ofsodium salts of carboxylic acids with different chain lengths hasbeen investigated. In this part a summary of the performances andlimitations of both mono- and di-sodium carboxylate inhibitors arepresented. For mono-carboxylates, the inhibition efficiency reacheda maximum value of 95% in stagnant aerated solutions at a chainlength of C=4 with a critical inhibition concentration of 6 mM in10 mM NaCl solution. However the inhibition efficiency graduallydecreased as the number of carbon atoms in the chain lengthincreased to more than 8, or less than 4, and this was in agreementwith surface hydrophobicity and contact angle results. For lowerchain lengths, the carboxylate anion becomes more acidic andcomplexing of the metal ion while for longer chain lengths, thecarboxylate anion becomes less soluble and tends to micellisewherby the active groups are no longer available for surfaceadsorption.For di-carboxylates the inhibition efficiency improvedin 10 mM NaCl at a given chain length compared withmono-carboxylates, and continued to increase to C=8 (sebacate),which achieved excellent inhibition efficiency. However, sebacateis costly so a blend with ethyl hexanoate was found to beeconomically favoured.Advisors/Committee Members: ZHOU, XIAORONG X, Lyon, Stuart, Zhou, Xiaorong.