The inhibition efficiency (IE) of an aqueous extract of henna leaves in controlling corrosion of carbon steel in seawater has been evaluated by weight-loss method. The weight loss study reveals that the formulation consisting of 8？mL of henna extract (HE) and 25？ppm of Zn2+ has 94% inhibition efficiency in controlling corrosion of carbon steel in sea water. Polarization study reveals that HE and Zn2+ system functions as mixed type inhibitor. AC impedance spectra reveal that protective film is formed on the metal surface. The nature of the metal surface has been analysed by FTIR spectra, SEM, and AFM analysis. 1. Introduction Sea water is one of the most corroded and most abundant naturally occurring electrolytes. The corrosive behaviour of seawater is reflected by the fact that most of the common structural metals and alloys are attacked by this liquid or its surrounding environments. The sea water environments can be divided into five zones, namely, subsoil, continuously submerged, tidal, splash zone above high tidal, and atmospheric zone [1]. The corrosion behaviour of metals and alloys differs from one zone to another. In splash zone the stainless steels have usually satisfactory performance while the carbon and low alloy steels do not. Anderson and Ross had found that the austenitic grades performed much better than martensitic and ferritic grades [2]. The Ni, Cu, and P alloyed steels were found to be much more resistant than carbon steel in splash zone [3]. Also, it was found that Mn, P, and Al had measurable influence on corrosion rates of low carbon steels under tidal exposure. After 5-year exposure test, it was found that the rate of attack in splash zone was much higher than the atmospheric and deep submerged zones [4]. Metals and alloys are often exposed to the action of acids and alkalis in industrial processes thereby prompting their deterioration [5]. One of the most effective means of protecting metals and alloys surfaces from corrosion in acid and alkaline environments is the use of corrosion inhibitors [6]. Corrosion inhibitors are usually added to the acid/alkaline solution to reduce the metal loss. Recently, studies on the use of drugs have been reported by several researchers [7, 8]. Some of these corrosion inhibitors are, however, toxic to the environment. This has prompted the search for green corrosion inhibitors that are nontoxic and ecofriendly for metals and alloys in acidic and alkaline solutions [9]. These green corrosion inhibitors have been found to have centre for -electrons and functional groups (such as –C=C–, –OR, –OH,
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