The Inhibitive Effect of Gnetum Africanum, Gongronema Latifolium and Chromolena Odaratum Extracts on Corrosion of Stainless Steel in 1 M HCl and H 2 SO 4 Solutions

. Extract of various plants; Okazi leaf (Gnetum africanum), Utazi leaf (Gongronema latifolium) and Elizabeth leaf (Chromolena odaratum) was investigated as corrosion inhibitor of stainless steel in 1M HCL and H 2 SO 4 using conventional weight loss. Results were obtained at intervals of 24, 48, 72 and 96 hours respectively. The corrosion rate and inhibition efficiency were calculated. The results revealed that all the plant extract act as inhibitors in the acid environments while Elizabeth leaf (Chromolena odaratum) proved to be better corrosion inhibitor than Okazi plant ( Gnetum africanum ) and Utazi plant ( Gongronema latifolium ) extract as it recorded the highest inhibition efficiency of 90.95% in 1M HCL for 20 ml concentration.


Introduction
Acid solutions are widely used in acid pickling and industrial cleaning. Hydrochloric acid and sulphuric acid are widely used for this purpose and most acid media cause metal corrosion [1,2,3]. Since aggressive acid solutions are widely used for industrial purposes, inhibitors are commonly used to reduce the corrosive attack on metallic materials. Corrosion inhibitors will reduce the rate of either anodic oxidation or cathodic reduction or both. This will give us anodic, cathodic or a mixed type of inhibition. Most of the potential corrosion inhibitors possess an active functional group, such as a nitro (-NO2) or a hydroxyl (-OH) [4,5] group. The inhibition of steel corrosion by acids was previously studied by some sulfur-and/or nitrogencontaining compounds [6]. A corrosion inhibition is a substance which when added in small concentration of an environment, effectively reduces the corrosion rate of a metal exposed to the environment [7]. In an attempt to find corrosion inhibitors that are environmentally safe and readily available, there has been a growing trend in the use of natural products such as leaves or plant extracts as corrosion inhibitors for metals in acid cleaning processes. Several corrosion inhibition studies of mild steel by plant extracts in acidic media has been carried out and reported. Hui et al. [8] has proved the use of herbs as new type of green inhibitors for acidic corrosion of steel. Elyn Amira et al. [9] have contributed significantly to the green mitigation by investigating several plants and their different body parts as corrosion inhibitors. Most researchers are focusing on natural products as corrosion inhibitors viz., Gossipium hirsutum L. [10], Cola acuminata and Camellia sinensis [11], Citrullus vulgaris Peel [12] and Rographis paniculata, Jatropha curcas [13], Stevia rebaudiana [14], Hibiscus sabdariffa calyx [15] Rosemary Extract [16] Eggplant Peel [17], Psidium guajava [18] Azadirachta indica [19] , Strychnos nuxvomica [20]. This study was done to evaluate the inhibitive effect of ethyl acetate extract of Gnetum africanum, Gongronema latifolium and Chromolena odaratum as a green corrosion inhibitor of stainless steel in aqueous solution. Okazi leaf (Gnetum africanum) is traditionally a wild vine and is considered to be a wild vegetable. It is widely distributed in Nigeria, Cameroun, Central Africa Republic, Equitorial Guinea and Gabon. This vegetable is important ingredients in preparing soup in many homes in the South-south and Southeast of Nigeria. Utazi leaf (Gongronema latifolium) is an herbaceous shrub, with flowers usually yellow and the stem yields characteristic milky exudates. It is commonly grown in Nigeria and is locally called utasi by the Efiks, Ibibios The assessment of the corrosion behaviour was studied using weight loss.

Material and Method
The steel rod was tested in the present study with a length of 50 mm and a diameter of 12 mm. The test aqueous solutions contained HCL and H 2 SO 4 with 1 M concentrations. Triple distilled water was used for preparing all solutions. In all measurements, mechanically polished electrode was used. Polishing was affected using successively finer grade of emery papers (600-1200 grade). About 25 g of dried and powdered leaves of each plant; Okazi leaf (Gnetum africanum), Utazi leaf (Gongronema latifolium) and Elizabeth leaf (Chromolena odaratum) were refluxed with ethanol for about 5 h and was kept overnight to completely extract the basic components. The insoluble residues were removed and the ethanol evaporated. The resultant material obtained was dried, powdered and weighed accurately by digital micro-balance. From the weight of extracted mass as above, different concentrations of 5, 10, 15 and 20 mL, respectively of the plant extracts were separately put into four beakers. The first beaker was kept plain, without any extract addition (control experiment). 20 pieces of the steel rod were then weighed for initial weight and immersed in each of the 1M HCL and H 2 SO 4 concentration. Four coupons were then suspended with rope in each of the beakers and were left for various time intervals (24, 48, 72 and 96 h, respectively). This process was repeated for the Utazi leaf (Gongronema latifolium) and Elizabeth leaf (Chromolena odaratum) extracts. The above procedure was also carried out for mild steel [21]. The weight loss was used to calculate the Corrosion Rate (CR) in milligrams per square centimeter per hour.

Results and Discussion
Okazi leaf (Gnetum africanum) in 1M H 2 SO 4 for stainless steel. The results obtained for the variation of weight loss with exposure time for the stainless steel specimens immersed in 1M H 2 SO 4 with varied concentrations of added okazi leaf (Gnetum africanum) extract is presented in Fig.1.     The curve specimen without any Gnetum africanum extract addition recorded the highest weight loss values, achieving 0.16, 0.30, 0.57 and 0.63g, after 24, 48, 72 and 96 hours of the experiment respectively. The addition of the suitable concentration of the Gnetum africanum extract shows significant reduction in the recorded weight loss values, which indicate the magnitude of corrosion. In this case, it may be concluded that a reasonable degree of corrosion inhibition of the test specimen in the acid medium was provided. All the four different concentrations of the Gnetum africanum extract had an initial better performance after 48 hours of the experiment, after that they all showed unstable performance after 72 and 96 hours of the experimental period.      fig. 9, it will be observed that 5ml, 10ml and 20ml concentration of Gongronema latifolium reduce from 24 hours of the experiment to 72 hours of the experiment and slight increase on the 96 hours of the experiment. While the 15ml concentration of Gongronema latifolium increase from 24 hours to 72 hours and then a reduction on the 96 hours of the experiment. The highest inhibition efficiency of 60.68% was recorded after 72 hours for the 15 ml concentration of Gongronema latifolium. The 15ml concentration of extract was recorded to be most effective followed by the 20 ml, 10 ml and 5 ml concentration of extract respectively.

Result for Utazi leaf (Gongronema latifolium) in 1M HCL for stainless steel.
The results obtained for the variation of weight loss with exposure time for the stainless steel specimens immersed in 1M HCL with varied concentrations of Utazi leaf (Gongronema latifolium) added extract is presented Fig. 10. The acid test medium with the four concentrations Gongronema latifolium of plant extracts addition in fig. 10 showed good weight loss inhibition effect of the immersed specimens, the four extracts concentration addition, achieved low weight loss values throughout the period of experiment. For most days of the experimental period, the extract with 20ml concentration addition to the test medium performed better than the 5ml, 10ml and 15ml concentration addition, though it achieved a comparative lower weight loss value of 0.01g after 24 hours, 0.03g after 48 hours and 0.1g after 96 hours of the experiment.

Result for Elizabeth leaf (chromolena odaratum) in 1M H 2 SO 4 for stainless steel.
The results obtained for the variation of weight loss with exposure time for the stainless steel specimens immersed in 1M H 2 SO 4 with varied concentrations of Elizabeth leaf (chromolena odaratum) added extract is presented Fig. 13.  In this case, it may be concluded that a reasonable degree of corrosion inhibition of the test specimen in the acid medium was provided. All the four different concentrations of the chromolena odaratum juice extract give very close values of corrosion rate after 72 and 96 hours. However, the 20ml addition had an initial better performance (6.35mm/yr CR) than the 15ml (6.88mm/yr CR), 10ml (7.29mm/yr CR) and 5ml juice (9.1mm/yr CR) additions.  15. The results obtained for the solution extract of chromolena odaratum at 20ml concentration gave very reasonable high inhibition efficiency at 58.03% for the 24hrs, and a reduction in efficiency down to 72 hours and a slight increase towards 96 hours, this phenomenon occurred for the 15ml concentration extract. The 10ml concentration has high inhibition efficiency at the 24 hours and then followed a decreasing trend to the end of the experiment showed, whereas the 5ml concentration has its highest value at the 72 hours and a lowest value at the start and end of the experiment.

Result for Elizabeth leaf (chromolena odaratum) in 1M HCL for stainless steel.
The results obtained for the variation of weight loss with exposure time for the stainless steel specimens immersed in 1M HCL with varied concentrations of Elizabeth leaf ( chromolena odaratum) added extract is presented Fig. 16. The test media without inhibitor has the greatest weight loss during the experiment compared other concentrations of inhibitors. It was observed that the media with inhibitors had great effect on the weight loss of stainless steel in 1M HCL acid. The weight loss of the different extracts of the inhibitor was within the same range after 24 hours. It was also observed that the medium with 20 ml concentration of the chromolena odaratum extract had the least weight loss and thus the best inhibitor used in 1M hydrochloric acid on stainless steel.  From fig. 18, the inhibitor efficiency of the 5ml, 10ml and 20ml showed an increasing trend from the 24 hours of the experiment up to 72hrs and then a decrease towards the end of the experiment. That of 15 ml which showed unstable inhibition efficiency throughout the experimental period. The highest inhibition efficiency from the graph is 90.95% was recorded at 72 hours for 20ml concentration of chromolena odaratum, while the least inhibition efficiency 57.30% was recorded at 24 hours for 5 ml concentration. The inhibition efficiency performance of the plant extract increased with increase in concentration used. Hence, it is concluded that the best concentration of the plant extract to use to obtain the best inhibition efficiency of the above medium was 20ml. Similar observations have been reported by Obiukwu et al [21] for the inhibitive effects of these plant extracts on mild steel in these acidic media. It is observed from comparison of the results that stainless steel has high rate of corrosion in H 2 S0 4 than in Hydrochloric acid (HCL) thereby giving a low and unstable inhibition efficiencies on stainless steel compare to mild steel [21] whereas the corrosion rate of both steels (mild and stainless) in HCL was at a pleasant rate which showed a higher and stable inhibition efficiencies on both steels. In 1M HCL acid on mild steel [21] and stainless steel, a maximum of 87.97% and 76.47% inhibition efficiency was obtained using 15 ml concentration and 20ml concentration respectively, providing low inhibition weight loss and corrosion rate. In 1M H 2 S0 4 acid on mild steel [21] and stainless steel, a maximum of 85.89% and 67.08% inhibition efficiency was obtained making use 20ml concentration and 10ml concentration respectively providing very low inhibition weight loss and corrosion rate. Among the various plant extracts used it was observed that Elizabeth leaf( chromolena odaratum) has the best inhibition efficiency on both mild steel [21] and stainless steel in both acidic media (HCL and H 2 SO 4 ) followed by Okazi leaf (Gnetum africanum) and Utazi leaf (Gongronema latifolium)which also showed good inhibition efficiency.

Conclusions
Gnetum africanum, Gongronema latifolium and Chromolena odaratum extracts showed inhibitive effect on corrosion of stainless steel in acidic environment. Inhibition efficiency increase with an increase in inhibitor concentration for the extracts. Elizabeth plant (chromolena odaratum) extract proved to be better corrosion inhibitor than Okazi plant (Gnetum africanum) and Utazi plant (Gongronema latifolium) extract since it has higher inhibition efficiency. From the experimental results obtained, the different plant extracts can acts as good corrosion inhibitor and thus be used to retards the corrosion rate of stainless steel if the appropriate concentration is used. It was observed that stainless steel has higher rate of corrosion in suphuric acid (H 2 S0 4 ) than in hydrochloric acid (HCL). Corrosion rate increases with time of exposure to the corrosive medium (inhibited or noninhibited was obtained for 20ml concentration and 10ml concentration. For 5ml, 10ml, 20ml, 20ml concentrations of Utazi leaf (Gongronema latifolium) plant extract at different time intervals, in 1M HCL acid, a maximum of 68.52% inhibition efficiency was obtained for 10ml concentration. In 1M H 2 S0 4 , a maximum of 60.68% inhibition efficiency was for 15ml concentration. For 5ml, 10ml, 20ml, 20ml concentrations of Elizabeth (chromolena odaratum) plant extract at different time intervals, in 1M HCL, a maximum of 68.50% and 90.95% inhibition efficiency was obtained for 15ml concentration and 20ml concentration respectively. In 1M H 2 S0 4 acid, a maximum of 58.03% inhibition efficiency was obtained with 20ml concentration.