An Investigation of Electrochemical Dechlorination of Wrought Iron Specimens from the Marine Environment

Abstract

The research shows the benefits provided by the use of electrochemical treatment, with the application of impressed current combining the use of a porous medium for the dechlorination of large iron structures removed and/or located in the marine environment. Considering the previous work for the dechlorination of the paddle wheel of the shipwreck “Patris”, located in the Aegean Sea, this study aims to determine the optimum parameters of the amount of current density, the time and the use of the porous medium to stimulate the chloride ion diffusion into an alkaline solution. Specimens of wrought iron coming from the shipwreck were electrochemically treated and the efficiency of the method was verified by the determination of the chloride concentration removal using a chloride ion selective electrode. Samples of corrosion products before and after treatment were analyzed for chloride content using SEM-EDX analysis. The results found that changing the porous medium every 24 h with replenished alkaline solution and using a stainless steel mesh is the best approach for the dechlorination of the specimens. This electrochemical method, is economical and fast, and can be applied to the conservation of large iron structures in situ, coming from and/or located near a marine environment with less waste than the traditional dehlorination methods.

1. Introduction

Structures from the marine environment are either structures located near the sea or shipwrecks, which usually are vessels or ships. Such marine finds are difficult to handle with conservation because of their size, and if they are removed from the sea without a plan of proper conservation for their stabilization, a rapid alteration from being wet to dry will occur, resulting in the destruction of those objects [1].

The marine environment responsible for the corrosion of ferrous structures near the sea has various zones. The splash zone, the tidal zone, and the wet–dry cycles determine the corrosion rate as well as the type and morphology of the corrosion products. Moreover, humidity and temperature play significant roles [2,3].

For shipwrecks, the corrosion rate mainly depends on factors of both the object and the undersea conditions. The design and composition of ferrous objects, the initial phosphorus inclusions, the carbon concentration in the alloy, and cracks on the object or surface roughness are factors of great importance [4], as well as the temperature, pH and salinity of the water, dissolved oxygen, water movement, waves, storms, shipwreck depth, and biological processes [5].

The massive concretions which completely cover the ferrous structures create a physical barrier between the object and the marine environment [6]). These concretions are responsible for the separation of anodic and cathodic reactions on the metal; the former is taking place on the metal’s surface and the latter inside the concretion layer. As a result of this, acidic conditions form at the metal’s surface due to hydrolysis of ferrous ions. The slag inclusions that wrought iron contains allow water to penetrate inside the metal due to electrochemical corrosion, resulting in a wood grain appearance and the loss of the original surface [7].

The predominate corrosion product for historical iron in the marine environment is β-Fe₂(OH)₃Cl. The β-form can locally be mixed with the γ- form of Fe₂(OH)₃Cl, creating a corrosion layer with a varying thickness from a hundred microns to a few millimeters. With the presence of oxygen, the β-phase becomes unstable and transforms, according to the pH and chloride concentration, either to magnetite Fe₃O₄ or to solid iron oxyhydroxides, goethite α-, akaganeite β-, and lepidocrocite γ-FeOOH. If the oxidation is rapid, akaganeite is the dominant phase [8]. Rapid oxidation happens by the time an iron object is excavated from the sea and allowed to be dried out. The corrosion layers crack because of their increased volume, letting more oxygen in to reach the metal’s surface [9].

The usual conservation methods aim for the extraction of chloride ions from the objects. Most of these treatments are based on the immersion of iron artifacts in high pH alkaline solutions [10,11,12]. The most common treatments of dechlorination and the stabilization of metallic objects include electrolysis, hydrogen plasma, and subcritical fluid, and the most common of all are simply the immersion in alkaline solutions and the alkaline reduction method [13,14].

What if the complete immersion of the object because of its size is not possible and the electrochemical treatment must be done in situ? The paddle wheel removed from the shipwreck “Patris” in 2008 in the Aegean Sea was allowed to dry out after its excavation, and its size could not be treated with the common dechlorination methods, due to the high economic cost [15]. The electrochemical chloride removal which took place for the dechlorination and stabilization of the paddle wheel was the technique of impressed current [16,17]. An impressed current is commonly applied by the industry to remove chlorides from iron rebars in porous materials, such as concrete [18,19,20]. The strength of this method regarding metal structures in the marine environment is that it can be used effectively, quickly, and in situ, with a small financial cost. The aim of this research is to determine whether the dechlorination, using DC current, of large marine metallic objects which have been left to dry is an effective method.

2. Materials and Methods

The electrochemical technique was applied to cylindrical (15 cm, average Ø 3.5 cm) wrought iron specimens (Figure 1) as presented in

Table 1. Dimensions of specimens.

Specimen	Dimensions (cm) (Length, Ø Diameter)
Reference (R)	15, Ø 3.4
3	15, Ø 3.4
4	15, Ø 3.5
5	15, Ø 3.4
6	15, Ø 3.5
7	15, Ø 3.5
8	15, Ø 3.6
9	15, Ø 3.5
10	15, Ø 3.6
11	15, Ø 3.5
12	15, Ø 3.4
13	15, Ø 3.4
14	15, Ø 3.4
15	15, Ø 3.4
16	15, Ø 3.5
17	15, Ø 3.5
18	15, Ø 3.4
20	15, Ø 3.5

. The specimens came from the shipwreck “Patris” located in the Aegean Sea. Particularly, an iron rod from the wreck was cut into isomer pieces of 15 cm with a special metal cutting machine at the School of Mechanical Engineering of Athens (N.T.U.A.). The dimensions of each specimen are presented in Table 1.

The porous medium was an industrial sponge made of polyurethane (Figure 2) (18′13′0.3 cm³ thick). It was tested for absorbency and resistance to 1 N sodium hydroxide (NaOH) solution in deionized water for at least 72 h, maintaining the pH of the alkaline solution above 13.

The used anodes (15′8′0.2 cm³) were type 316 stainless steel.

The alkaline solution was 1 N NaOH in deionized water.

The average temperature during the experimental procedure was 20 °C (laboratory temperature).

Before the electrochemical treatment, the specimens were totally washed for ten minutes with deionized water (with a wash bottle) in order to remove soluble salts and dust. In each experimental procedure, an amount of 1 N NaOH solution was taken from the porous medium (when used) or the solution of the system to determine the chloride ion removal. The same amount of 1 N NaOH, 50 mL, was added to the porous medium for each procedure except for the specimens 14, 15, 16, and 18 (as given in

Table 2. Electrochemical treatment of wrought iron specimens.

Specimen	Porous Medium	Volume of Alkaline Solution 1N NaOH (mL)	Current Density (mA/cm2)	Voltage (mV) SSE	Time (h)
Reference R	-	-	-	-	-
3	Sponge (same)	350	1	−980 to −900	72
4	Sponge (same)	350	5	−980 to −900	72
5	Sponge(same)	350	7.5	−980 to −900	72
6	Sponge (same)	350	10	−980 to −900	72
7	Sponge (same)	350	15	−980 to −900	72
8	No sponge	1000	1	−980	72
9	No sponge	1000	5	−980	72
10	No sponge	1000	7.5	−980	72
11	No sponge	1000	10	−980	72
12	No sponge	1000	15	−980	72
13	No sponge	1000	8	−795 to −835	72
14	Sponge	180	4	−880 to −840	24
15	Sponge	180	4	−890 to −680	48
16	Sponge	180	4	−820 to −660	72
17	Different sponge every 24 h	350	8	−785 to −690	72
18	Sponge	350	8	−760 to −700	72
20	Sponge and mesh	350	8	−760 to −680	72

). After 24 h, 50 mL of the solution was removed from the sponge and, if the procedure was continued, 50 mL was replenished. The wrought iron specimen was connected to the negative pole and the stainless steel anode to the positive pole with cables and crocodile clips to a transformer/rectifier (24 V, 10 A). A mercury/mercurous sulfate (Hg/Hg₂SO₄) reference electrode was placed into the porous medium to measure the potential voltage between the anode and cathode of the system. During the procedure, the voltage potential of the SSE was less negative than −1000 mV (as given in Table 2) so as to avoid hydrogen embrittlement [21]. After the treatment, each specimen was rinsed with deionized water and dried in an oven at 115 °C for 40 min, and then samples of corrosion for each layer specimen were taken for Scanning Electron Microscopy—Energy Dispersive X-ray (SEM-EDX) analyses.

The concentration of chloride ions of the alkaline solution was determined with a chloride ion-selective electrode (HANNA ISNSTRUMENTS HI 98172N with HI 4107 electrode). Specifically, the samples of alkaline solution of the porous medium, before the use of the selective electrode, were processed as follows: 50 mL of the alkaline solution was taken from the porous medium and was neutralized with 25 mL 1 N nitric acid (HNO₃) solution, then 1 mL of HANNA ISA for electrode solutions and 1 mL buffer solution were added. The buffer solution was prepared by dissolving 19.5 g ammonium acetate (C₂H₄O₂·H₃N) and 15 mL acetic acid (CH₃COOH) in 200 mL of deionized water and diluting the solution to 250 mL. The final solution of the sample was stirred using a magnetic stirrer for at least 15 min. Before each measurement, the reliability of the selective electrode was checked according to its worksheet.

The SEM-EDX analyses of the samples 6’3 cm² in size were taken from the corrosion products made by a type JEOL JSM-6510LV with X-act.

Table 2 briefly presents the electrochemical treatment of each specimen.

For all the experimental measurements of each specimen, the concentration of the chloride ion removal for 48 and 72 h treatments was added to the previous, except for the specimens 14, 15, and 16.

3. Results and Discussion

3.1. Electrochemical Treatment

Five specimens (3, 4, 5, 6, and 7) were wrapped with the sponge containing 1 N NaOH solution. The current densities of 1, 5, 7.5, 10, and 15 mA/cm² were applied with an average applied voltage of the SSE (−980 mV to −900 mV) for the 72 h electrochemical treatment. The total amount of chloride ions removed from the specimens after 24, 48, and 72 h are shown in Figure 3 below.

Five more specimens (8, 9, 10, 11, and 12) were treated with the electrochemical method with the same current densities and a constant voltage of the SSE (−980 mV) for the same amount of time. However, the specimens were not wrapped with a sponge, but were immersed in the solution. The chloride ion removal versus time is shown in Figure 4 below.

The delay that appears in Figure 3 and Figure 4 could be related to the diffusion coefficient of the chloride ions in the corrosion layers of the specimens (see Selwyn et al. 2001) and/or the use of the porous medium (for specimens 3, 4, 5, 6, and 7) [22].

Three wrought iron specimens (14, 15, and 16) were treated with the same electrochemical procedure. Specimen 14 was wrapped with sponge and was treated for 24 h, with specimen 15 being treated for 48 h, and specimen 16 for 72 h. The whole procedure for each specimen was uninterrupted for the duration of the treatment. The applied impressed current was stable at 4 mA/cm² and the average applied voltages of the SSE for the specimens were −860 mV, −850 mV, and −735 mV, respectively. The concentrations of the chloride ion removal for specimens 14, 15, and 16 are seen in

Table 3. The concentrations of chloride ion removal for specimens 14, 15, and 16.

	Chloride Ion Concentration
Specimen	24 h	48 h	72 h
14	294.98	-	-
15	-	439.71	-
16	-	-	478.45

below.

Specimen 13 was treated with the electrochemical method in complete immersion in 1 N NaOH, without a sponge, for 72 h. Specimen 17 was electrochemical treated with a sponge, which was replaced by a new one every 24 h. After the 24 h treatment, the specimen was rinsed with deionized water and dried in an oven at 115 °C for 40 min. The specimen was wrapped with a new sponge and the electrochemical procedure was started again for another 24 h. Specimen 18 was treated for 72 h with a sponge. The difference in the method for specimen 20 from the other specimens is that the anodes were stainless steel mesh. Specimens 13, 17, 18, and 20 were carried out according to Table 2.

The comparative results from the dechlorination of specimens 13, 17, 18, and 20, which were treated with the same current density of 8 mA/cm² but used different procedures concerning the porous medium, are presented to the Figure 5 below.

Chloride ion diffusion into the alkaline solution was higher for the specimens that were not wrapped with a porous medium (complete immersion). Specimen 17 had a higher chloride ion concentration with the alkaline solution compared to the other specimens (18, 20) with a porous medium. Specimen 20 had higher chloride ion removal compared to specimen 18 after 72 h treatment.

3.2. Determination of Final Chloride Concentration with HNO₃

The total concentration that remained for specimens 13, 14, 15, 16, 17, 18, and 20, as well as the specimen R (Reference), which was not treated at all, was determined using a sample of 2 g of the corrosion products which was dissolved with 50 mL in a solution of 5 N HNO₃ (65%) [23] for 48 h, as seen in

Table 4. Chloride ion concentration before (specimen R) and after the dissolution of corrosion products of the specimens into a 5 N HNO₃ solution.

	Specimen
	R	13	14	15	16	17	18	20
Chloride ion concentration (ppm)	601	234	478	445	399	329	352	336

.

Below,

Table 5. The difference of chloride ion concentration measured by the dissolution of the corrosion products into 5 N HNO₃ before and after electrochemical treatment.

	Specimen 13	Specimen 14	Specimen 15	Specimen 16	Specimen 17	Specimen 18	Specimen 20
Chloride ion concentration (ppm)	367	123	156	202	272	249	265

shows the efficiency of the electrochemical method. When a porous medium is used, the results are better when the sponge is replaced every 24 h (Specimen 17) than when the sponge is used for the duration of the treatment 72 h (specimen 18 and 20).

3.3. Factors of Chloride Ion Diffusion

According to a theoretical approach, the chloride ion diffusion into the alkaline solution can be represented as follows:

$$[{Cl}^{-}] = \frac{I\times t\times M}{n\times F}$$

(1)

where

• I: current density;
• t: time;
• M: molecular weight of chloride;
• n: number of electrons on the chloride ion;
• F: Faraday constant.

The theoretical concentrations of chloride ion diffusion [Cl⁻]_th into the alkaline solution for the applied current densities are presented in

Table 6. Theoretical concentration of chloride ion diffusion into alkaline solution.

Current Density (mA/cm2)	Theoretical Concentration of Chloride Ions Diffusion (g)
	24 h	48 h	72 h
1	0.032	0.064	0.095
4	0.127	-	-
	-	0.254	-
	-	-	0.381
5	0.159	0.317	0.476
7.5	0.238	0.476	0.715
8	0.264	0.508	0.762
10	0.317	0.634	0.953
15	0.476	0.952	1.429

.

The theoretical concentration of chloride ion diffusion increases with time and the applied current density.

The factor (α) of chloride ion movement to the alkaline solution would be as follows:

$$\alpha =\frac{{\left[C{l}^{-}\right]}_t}{{\left[C{l}^{-}\right]}_{th}}$$

(2)

where

• [Cl⁻]_th: the theoretical concentration of chloride ion extraction;
• [Cl⁻]_t: the experimental concentration of chloride ions in the alkaline solution for 24, 48, and 72 h.

The factor α, for the applied current densities, is briefly presented in

Table 7. Chloride ion movement (1 − α) into the alkaline solution during the electrochemical treatment.

Specimen	Current Density (mA/cm2)	Porous Medium	1 − α 24 h	1 – α 48 h	1 – α 72 h
3	1	Sponge	0.102	0.387	0.534
4	5	Sponge	0.758	0.851	0.900
5	7.5	Sponge	0.849	0.899	0.929
6	10	Sponge	0.830	0.909	0.930
7	15	Sponge	0.850	0.921	0.943
8	1	No sponge	0.023	0.164	0.111
9	5	No sponge	0.403	0.680	0.752
10	7.5	No sponge	0.590	0.747	0.825
11	10	No sponge	0.609	0.789	0.858
12	15	No sponge	0.685	0.840	0.881
13	8	No sponge	0.610	0.744	0.784
14	4	Sponge	0.429	-	-
15	4	Sponge	-	0.567	-
16	4	Sponge	-	-	0.686
17	8	Different sponge	0.716	0.759	0.811
18	8	Sponge	0.723	0.824	0.870
20	8	Sponge and mesh	0.862	0.858	0.846

.

From Table 7, it is concluded that, when the amount of the applied current density is low, the chloride ion removal is carried out in an inversible way [24,25]. Through the determination of 1 – α, the results in Table 7 follow the results of Figure 3 and Figure 4 and Table 3.

3.4. SEM-EDX Analysis

Firstly, a sample of the corrosion layer of specimen R (Figure 6) before and after its treatment with 5 N HNO₃ solution was analyzed with a Scanning Electron Microscope. The concentration (% wt) of chloride ions before and after the treatment is seen in

Table 8. Chloride ion concentration of corrosion products of the specimens according to SEM-EDX analyses.

Specimen	Average Chloride Ion Concentration % wt of the Points A, B, C, D	Time (h)
R (initial chloride ion concentration)	4.61	0
13(I = 8 mA) no sponge	1.71	72
14 (I = 4 mA) sponge	3.69	24
15 (I = 4 mA) sponge	3.38	48
16 (I = 4 mA) sponge	3.20	72
17 (I = 8 mA) different sponge	2.40	72
18 (I = 8 mA) same sponge	3.07	72
20 (I = 8 mA) sponge and mesh	2.67	72
R (5 g) after dissolution with 5 N HNO3	0.13	48

, where A = 0.5 mm, B = 1.5 mm, C = 2 mm, D = 3 mm are the distances between the points in the layer of the corrosion products from the internal surface (metal/oxide) to the external surface (oxide/environment).

Figure 7 presents the comparative results for the specimens R, 15, and 16, according to SEM-EDX analysis.

Figure 8 presents the comparative results for the specimens R, 13, 17, 18, and 20.

The factor F, according to the average chloride ion concentration of points A, B, C, and D for each specimen, from SEM-EDX analysis, is given in

Table 9. Factor F of dechlorination of specimens according to the experimental results from SEM-EDX analysis.

Specimen	Treatment	Factor (F)
13	No sponge	0.630
14	Sponge	0.200
15	Sponge	0.266
16	Sponge	0.305
17	Different sponge	0.479
18	Same sponge	0.334
20	Sponge with mesh	0.419
R reference after treatment with 5 N HNO3	Treatment with 5 N HNO3	0.972

. It must be noted that, for all the below measurements, to ensure comparative results, it was assumed that the initial chloride ion concentration for all specimens was the one found for the specimen R (Reference), according to SEM-EDX analysis. For all the specimens, it was assumed that the chloride ions were uniformly distributed.

Electrolysis with a porous medium was inferior to the complete immersion of the specimens for chloride ion removal, but it also involves the use of less volume of alkaline solution, which is important for chloride ion diffusion. The remaining concentration of chloride ions for specimen 18, which was treated with the same sponge for 72 h, after electrolysis, was higher than that of specimen 17, whose sponge was changed every 24 h. Changing the porous medium every 24 h offers a fresh alkaline solution for the chloride ions to diffuse into. Specimen 20 was treated for 72 h with sponge and mesh anodes; according to SEM analysis, it had almost the same remaining chloride ion concentration as specimen 17 after electrolysis.

3.5. A Mathematical Model for Extraction Ratio of Chloride Ions

For the estimation of the extraction of the chloride ions of the specimens, a mathematical model was used. According to Quyang et al. (2009) [26] research on marine cast iron artifacts, this equation represents the total number of chloride ions initially in the iron object:

N_o = (C_f⁰ + C_b⁰) × A × D

(3)

where

• C_f⁰: the initial concentration of the free chloride;
• C_b⁰: the initial concentration of the binding chloride;
• A: the area of the object;
• D: the length of the object.

For the specimens used in the experimental procedure of the present research, the average length and area of the specimens were A = 184.08 cm² and D = 15 cm. For the above mathematical equation, according to the authors, it was assumed that the volume of the treatment solution was large enough to consider that the concentration of chloride ions extracted into solution was negligible, compared to the one of the corrosion layer. In the present research, the same assumption was made. The C_f⁰ was known from the experimental procedure (Figure 3 and Figure 4 and Table 3). Furthermore, it was made the assumption that the chloride ions were uniformly distributed for each specimen.

These are the concentrations of the chlorides that remained (4) and that were extracted (5) from the object, as a factor of the treatment time, according to the research:

Ν_in(t) = C_b × A × d

(4)

Ν_out(t) = Ν_ο − Ν_in

(5)

The division of the above Equations (4) and (5) gives the extraction ratio of the chloride ions removed from the specimens. The extraction ratio of chloride ions is given by Equation (6) and is presented in

Table 10. The extraction ratio of chloride ions.

Specimen	Current Density I (mA/cm2)	Volume of Alkaline Solution (mL)	Treatment	Extraction Ratio %
13	8	1000	No sponge (72 h)	63.1
14	4	180	Sponge (24 h)	19.9
15	4	180	Sponge (48 h)	26.5
16	4	180	Sponge (72 h)	30.5
17	8	350	Different sponge every 24 h	47.1
18	8	350	Sponge (72 h)	33.4
20	8	350	Sponge and mesh (72 h)	41.9

.

$$\frac{N_{in\left(t\right)}}{N_{out\left(t\right)}}\%$$

(6)

From Table 10, it can be concluded that the extraction ratio deals with the amount of current density, the volume of alkaline solution, the use of porous medium, the type of anodes, and the time of treatment. For specimens 14, 15, and 16, which were treated with the porous medium with the same current density and volume of the alkaline solution, but for different amounts of time, using the extraction ratio, it is concluded that chloride ion removal is reduced after 48 h of treatment. For specimens 17 and 20, it is obvious that changing the porous medium every 24 h leads to better dechlorination results, as does the use of mesh instead of plates. The use of mesh for the anodes leads to tighter wrapping of the specimens with the sponge, making all the areas of the specimen are connect with the anodes, something that is not possible when using plates as anodes.

The above results are the same as those from the concentration of chloride ions in alkaline solution, the dissolution of the samples of the specimens in 5 N HNO₃ solution, and the SEM-EDX analyses. If a porous medium is used for the dechlorination of the specimens, the efficiency of the method (chloride removal) is increased when the porous medium is changed every 24 h.

4. Conclusions

The aim of the research is the electrochemical dechlorination of wrought iron specimens that have been exposed to the marine environment for a long time period. During the experimental procedure, the specimens were electrochemically treated with and without a porous medium (sponge), which was saturated with 1 N NaOH. Considering the electrochemical treatment, the following can be concluded:

• Marine iron objects can be electrochemically dechlorinated with the use of a porous medium with sodium hydroxide solutions;
• The use of a sponge is inferior for the removal of chloride ions versus the complete immersion in sodium hydroxide solutions;
• When a porous medium is used, a lower volume of alkaline solution is required than complete immersion;
• Changing the porous medium every 24 h increases chloride iron removal compared to using the same sponge during the electrochemical treatment;
• The performance of electrochemical method improves as the current density increases.

This work shows the benefits provided by the use of electrochemical treatment with the application of an impressed current, combined with the use of porous material for the dechlorination of large iron structures from the marine environment. However, further experimental research is needed to determine the additional parameters for the proposed method, in order to achieve the fast, economic, and effective dechlorination of metallic structures coming from or located near the marine environment.