The phenomenon of copper corrosion in seawater was studied at four different marine stations: the first of them is the beach of Agadir, the second is industrial zone Anza, the third is the port, and the last is the Aghroud zone (Figure 1). These sites attempt to be representative of the seawaters of Agadir, and different in the type of industrial activity and wastes. Table 1 reported the values of some physic-chemical parameters of the seawater collected.

The samples were prepared using copper 99.99% (weight percent), which was cut from a rectangular copper rod, with a total area of 5.68 cm² for the electrochemical and gravimetric tests. Before the electrochemical measurements, the surface of copper was abraded using different grades of sand papers, which ended up with the 1200 grade. Then, the electrode was cleaned by washing with distilled water, acetone, distilled water, respectively, and immersed into the test solution quickly. The microbiological influenced corrosion was studied using a selective medium named Starkey medium in order to look for the reducing sulfate-bacteria culture. This medium is composed by: Na₂SO₄ (4 g), MgSO₄∙7H₂O (2 g), NH₄Cl (2 g), KH₂PO₄ (0.5 g), yeast extract (1 g), Fe and Ca (traces), sodium Lactate (60%) (10 ml) and distilled water (1000 ml). The pH of this media was adjusted at 7.2 by the addition of 10 M KOH solution. After that the medium is sterilizing at 121˚C for 15 min.

A conventional three electrode cell was used for all the electrochemical measurements. A saturated calomel electrode (SCE) was used as a reference electrode, platinum electrode acts as a counter electrode and the test material as the working electrode. Natural sea water collected from the coastal area of Agadir and synthetic seawater, at different concentration of sulfide ions, served as the electrolyte. Open circuit potential (OCP) and EIS measurements were carried out using Volta lab PGZ 301 Electrochemical Analyzer under computer control. The EIS experiments were realized in the frequency range from 100 kHz to 10 mHz at E_ocp.

The charge-transfer resistance (R_t) values are calculated from the difference in impedance at lower and higher frequencies, as suggested by Tsuru et al. [21] . The double layer capacitance (C_dl) and the frequency at which the imaginary component of the impedance is maximal (−Z_max) are found as represented in Equation (1):

where (1)

In order to test the reproducibility, the experiments were performed in triplicate. Cyclic voltametry was carried out for copper electrode in the natural seawater solution. The working electrode is scanned from negative to positive values in the potential range of −600 mV to 400 mV at a scan rate of 20 mV∙s⁻¹. Additionally, other samples are prepared for the Atomic absorption measurement in order to follow up the evolution of the copper ions in the seawater solution at different durations. In this work, we are reporting to studying the corrosion of copper in the sea water in a long term. The maximal during is three months. After each 15 days, we proceed to the bacterial, electrochemical and gravimetric tests to follow up the corrosive behavior of copper and its dissolution in the corrosive medium. The corrosion rate is calculation by:

where Δm is the average weight loss before and after exposure, respectively; S is the surface area of sample and t is the exposure time.

Immersion corrosion analysis of copper sample in the natural seawater solutions was performed using optical microscopy (OM). Immediately after the corrosion tests about three months, the samples were subjected to OM studies to examine the surface morphology. OM East Scope was used for the experiments. The working sample was analyzed at three different locations to ensure reproducibility.

The results of OCP variations are shown in Figure 2. In all marine zones, the OCP values associated to copper shifted to negative ones, but there are some differences which appear by increase in the immersed time. As seen in Figure 1, the OCP values associated to copper in Z1 seawater are more negatives, they begins at −230 mV and shifted at the end into −353 mV. However, the OCP values of copper in Z4 seawater are noble witch variated between −235 mV and −186 mV. But the copper takes a similar OCP values in Z2 and Z3 seawaters, they are comprise at −200 mV and −300 mV. Decrease in OCP is an indication of alloy nobleness deterioration and surface activity acceleration [22] .

EIS was used to investigate the electrochemical properties of the corroded surface after immersion of copper in naturel seawater of different marine zones for 15 days. This technique leads also to compare the corrosion rate of copper in different immersion period and studied marine zones. In fact, the corrosion rate is proportional to the inverse of the R_t (1/R_t). Figure 3 presents the Nyquist diagram of copper immersed in natural seawater and the electrochemical parameters issues form EIS measurements are given in Table 2.

We observe that the resistance to transfer of charge changed with the marine zones. It takes a value of 18.81 kΩ.cm² for the Z1, 11.25 kΩ∙cm² for Z3, 9.27 kΩ∙cm² for Z2 and7.808 kΩ∙cm² for Z4. Consequently, after 15 days of immersion, the copper surface was modified: the resistance to transfer of charge of copper in the zones 2 and 4 is less than in 1 and 3 zones. This result can be explicated considering that the charge of bacterial didn’t have yet a enough time to develop in the medium and the surface was covered by the corrosion products or substances containing in the corrosive media which form a film on the metallic surface. Table 3 and Figure 4 present electrochemical parameters issues from the impedance response and the corrosion rate obtained by atomic absorption of copper samples exposed to seawater media of four marine zones and their evolution with time for three months as a maximal immersion period.

The graph presented in Figure 4 is associated to the copper corrosion behavior in Z1 seawater; it appears that the inverse resistance to transfer of charge, calculated from the impedance diagrams, increases at the first 30 days to decreases after the second month and starts to increase for the last month and it appears that the corrosion rate follows the same evolution. This results means that in this marine zone, firstly the copper was dissolved in marine environment but after one month the metallic surface is covered by the corrosion products which protect the metal. However, by the comparison of the evolution of electrochemical and gravimetric parameters in Z2 seawater (Table 3), the inverse of resistance to transfer of charge increases at the first month, and after this period it decreases progressively until the end of the exposure time [23] - [28] . This is the industrial port of Agadir city, so it receives a considerable quantity of polluted substances and hydrocarbons issues from different activities and results of deterioration of materials immersed in the seawater of port zone.

As will be discussed, this maximal decrease in the corrosion rate value is attributed to the formation of a thick layer of corrosion products at the surface with a large amount of fouling to form a composite material. For short immersion times the metal surface is covered by a thin layer of copper oxide (Cu₂O) of dark brown color together with an increasing coating of green patina [29] . The third column of the table 3is attributed to copper immersed in Z3 seawater. It shows that the corrosion rate and inverse of the resistance to transfer of charge are very maximal at the second month of immersion. This graph describes the same evolution of growth cycle of sulfate-reducing bacteria (SRB) obtained in Anza zone for three month of immersion (Figure 5). In this zone, the corrosion process is controlled by the microbiology influenced corrosion MIC, provokes by the SRBs. Those bacteria are able to reduce the sulfate contained in seawater environment into sulfide and to product extracellular

polymeric substances (EPS) which leads to the dissolution of the copper [29] . This mechanism needs a time to the bacterial proliferation and metabolic stimulation. The last column in Table 3 shows that the corrosion rate and the inverse of resistance to transfer of charge the copper immersed in Z4 seawater present two maximal peaks: at the first and second months of immersion. The corrosion rate evolution in this zone is different than other zones because the values of the corrosion rate for Z4 seawater are less. It is the reason of the appearance of two maximal peaks of the rate. In addition, the copper surface immersed present a few corrosion products, and it isn’t enough covered and protected by the patina layer. All this results are compatible with the nature of each zone and its pollution level and showed that the biofilm produced by the sulfate-reducing bacteria accumulates with the exposure time especially after two month of exposure [30] .

Figure 5 translates the evolution of corrosion rate of copper estimated by the both methods electrochemical and gravimetric in different marine environment after 15days of under immersion conditions. We notice that we have a good correlation between the results obtained by the methods used. We observe the corrosion rate and the inverse of the charge transfer are more important in Z4 and Z2.

Cyclic voltammograms (CV), copper electrode in natural seawater at 20 mV∙s⁻¹ are presented in Figure 6 and Table 4. It can be observed that two oxidation peaks in forward scan and one large reduction peak in the reverse scan. The peak a₁ is related to the formation of CuCl salt layer [31] . However, the second oxidation peaks (a₂) corresponds to oxidation of Cu⁺ into Cu²⁺. The large reduction peak corresponds to the reduction of soluble complex and the CuCl layer formed on the copper surface. By the analysis of potential and current values of CV curves, we observed that the first oxidation of Cu to Cu⁺ is important in the Z3 compared to other zones. It is in agreed with the microbiological tests revealed that this zone was polluted by the sulfate-reducing bacteria which reduces the sulfate to sulfide which participate at the metal dissolution.

The microbiological analysis concerns the count of sulfate-reducing bacteria in the seawater of Agadir coastal. The results of this analysis are represented in Figure 7 and Table 5. Seawater samples are collected from four different marine areas.

The microbiological tests realized shows that the Beach, the Port and Aground zones present the absence of all kind of sulfate-reducing bacteria. However, the Anza zone shows the present of a considerable charge of sulfate-reducing bacteria as known as the seawater is collected near than an area when is rejected the waste water of the city. This charge varies with the immersion time, it increases when the immersion begins until 75 days and it decreases after. The growth takes a maximal value at the 60th day. This evolution is the cycle of the life of this bacterial group. It is logically that this group acquires enough time to adapt with the environment on searching different sources of carbon, sulfate, sodium and other important elements for its growing and development. After this step, it moves to attain maximal proliferation at optimal conditions (pH and elements) by consuming the elements containing in the medium which will be exhausted to decrease the bacterial charge [31] [32] .