The effect of selected plant extracts on the corrosion behavior of duplex stainless steels

Duplex stainless steels are often selected for applications requiring mechanical strength in medium corrosive environments, such as the chemical and petrochemical process industries, paper production and desalination plants. The specific advantages offered by duplex stainless steels over the conventional 300 series austenitic stainless steels are improved strength, chloride stress corrosion cracking resistance and pitting corrosion resistance. Due to the overall cost and environmental implications caused by corrosion failures, several methods can be adopted to reduce the risk of corrosion by materials. One of the most practical methods for protection against excessive metal dissolution is the use of a proper inhibitor. Corrosion inhibitors are substances which, when added in small concentrations to corrosive media, decrease or prevent reaction of the material with the media by either reducing the probability of corrosive attack or reducing the rate of attack, or both. Inorganic substances such as phosphates, chromates, dichromates and arsenates have in the past been used as effective inhibitors, but suffer from the disadvantage that some of them are toxic and all of them have severe deleterious effects when released into the environment. This has lead to a search for alternative corrosion inhibitors. Organic substances containing polar functional groups with oxygen, nitrogen and/or sulphur atoms in a conjugate system have been reported to exhibit good inhibiting properties. However, not all of them are non-toxic and many such compounds are rather expensive. Consequently plant extracts were investigated as alternatives and a number were found that exhibited good corrosion inhibiting properties. Furthermore, they have the advantage of being environmentally friendly and acceptable, readily available, renewable and sustainable. Although a number of investigations have recently being reported in the technical literature, none of them considered duplex stainless steels. It was therefore decided to investigate the effect of selected African plant extracts as corrosion inhibitors for duplex stainless steels in hydrochloric and sulphuric acids. This paper will describe the results obtained from electrochemical evaluations. It was found that honey was the best inhibitor for type 2205 and 2507 duplex stainless steels in sulphuric acid from all the compounds evaluated. A concentration of 5g/l of honey was required to observe an inhibition of metal dissolution in the corrosive acidic media. Mugwort oil was the best corrosion inhibitor in hydrochloric acid for both types duplex stainless steels. Most of the plant extracts acted as mixed inhibitors, except for eucalyptus oil which acted as an anodic inhibitor for 2507 in hydrochloric acid.