Identification of sample
The weathering of the iron relics is observed underwater. The result of the identification of the sample is shown in Fig. 4. Fig. 4. shows the weathering and corrosion that occurs on an iron cannonball material through the formation of concretion (a buildup of crust), and the damages cause the breaking and destruction of the objects. Fig. 4a shows the iron metal that undergoes weathering and a low level of corrosion. Fig. 4b shows the iron metal that undergoes weathering and corrosion, and cracks are present on the ferrous objects. Fig. 4c shows the iron metal that undergoes weathering and corrosion, where large amounts of iron suffered from splitting. Fig. 3d shows iron metal that undergoes weathering and corrosion, exhibiting advanced damage, which gives rise to rupture and the destruction of the iron objects.
When iron is exposed to the atmosphere, the environment forms different iron-oxides, such as magnetite (Fe3O4), hematite (α-Fe2O3), and maghemite (γ-Fe2O3) [3]. At temperatures higher than 560 °C, the general sequence of the iron-oxide layer (from the interior to each surface) is Fe/FeO/Fe3O4/Fe2O3/O2 [4]. The redness of the rust powder and the presence of many cracks and cavities on the surface of the object are indicative of an active corrosion process being in progress, causing the continuous loss of metals, as well as the degradation of the mechanical properties [5]. The corrosion of iron-based archeological artifacts immersed in seawater is an electrochemical process that involves anodic and cathodic reactions in an aqueous electrolyte environment, and biological processes also involve anaerobic bacteria [2]. When iron is put into solution, the oxide layer grows slowly, forming oxide compounds, such as goethite (α-FeOOH), akageneite (β-FeOOH), and lepidocrocite (γ-FeOOH) [3, 6, 7, 8, 9, 10, 11, 12].
Characterization of the surface corrosion of iron cannonball materials immersed in water using a handy microscope
The results of the analysis and characterization of the objects, obtained using a handy microscope, are shown in Fig. 5. Fig. 5. can show the presence of corrosion on the immersed iron cannonball, and the corrosion processes can be distinguished into two types, namely, dry active corrosion, as shown in Fig. 5a and wet active corrosion, as shown in Fig. 5b.
The ongoing problem with iron archeology is the continued corrosion that occurs after excavation, which is caused by salt accumulation during burial. One way to repair iron cultural heritage material is by immersing the objects in a solution and waiting for chloride ions to spread out [5]. The weathering of underwater relics generally takes place faster than that of land-based relics. The rate of the weathering of cultural objects immersed in water can be 5-10 times faster than that of cultural heritage objects on land [1]. The speed of weathering is a result of the interaction of the material with water containing salt and exhibiting a biological activity. Chemical weathering reactions occur quickly in an aqueous medium because the reaction takes place effectively.
XRD characterization of the surface corrosion of iron cannonball material immersed in water
The result of the characterization of the surface corrosion using XRD has been shown in Fig. 6a and Fig. 6b. X-ray spectrometry methods such as XRD, XRF, and SEM-EDX/EDS are very suitable for the analysis of inorganic material in the field of conservation and heritage restoration [13, 14, 15, 16, 17]. Before carrying out conservation, the material to be conserved must be examined, so it is more appropriate to determine conservation techniques by considering the costs and resources [18].
The X-ray diffraction patterns in Fig. 6 clearly show the distinction of the compounds contained in the corroded material. As shown in Fig. 6a, peaks are observed at 2θ positions of 26.67° and 35.11°, while Fig. 6b shows peaks at 2θ positions of 26.67° and 35.17°. These peaks correspond to akageneite, as supported by the research of Gil et al. [19], who found the presence of akageneite to correspond to the peaks at 2θ = 26.68° and 2θ = 35.18°. The X-ray diffraction results indicate that the sample obtained from the corroded iron contains two types of iron oxide, akageneite and lepidocrocite.
The dominant phases in the iron artifacts are goethite (α-FeOOH) and magnetite (Fe3O4). The presence of these material types in the corrosion products explains the good preservation of the base metal (iron) for centuries and the stability after excavation. The corrosion product of iron-containing chloride ions, for example, is akageneite [20]. In addition to artifact materials, the corrosion process is influenced by environmental pollutants, other archeological materials, geography, the microorganisms in the soil, vegetation, land use, soil chemistry, soil physical properties, and the presence or absence of water and air [5].
Fig. 6 shows the X-ray diffraction results obtained for the iron cannonball sample corroded by wet corrosion, which was shown to form four types of iron oxide, i.e., halite (NaCl), akageneite, briartite, an iron mineral that is a gray metallic opaque sulfide, namely, Cu2 (Zn, Fe) GeS4, and famatinite (copper, antimony, and sulfur), which is a pink-brown mineral containing copper, antimony, and sulfur.
Various artifacts found on the shipwreck, including small arms and ammunition, show their involvement in naval battles [21]. The results showed that the cannonball was made of iron and was produced with sand casting molds. The sand found in the cavities of the cannonballs were also studied by petrography [22]. The mineral composition was obtained from the XRD characterization. The result of the mineral composition investigation is shown in Table 1.
Table 1
Characterization of surface corrosion using XRD
No.
|
Dry active corrosion
|
Wet active corrosion
|
Mineral
|
Amounts (%)
|
Mineral
|
Amounts (%)
|
1
|
Halite
|
9.12
|
Akageneite
|
96.68
|
2
|
Akageneite
|
89.63
|
Lepidocrocite
|
3.32
|
3
|
Famatinite
|
0.72
|
-
|
-
|
4
|
Briartite
|
0.53
|
-
|
-
|
The iron mineral is akageneite (III) oxide hydroxide/chloride, with the formula Fe3+O (OH, Cl). However, lepidocrocite, also called esmeraldite or hydrohematite, is a natural occurring iron oxide-hydroxide mineral with the formula γ-FeO(OH). Table 1 shows the presence of minerals in the form of akageneite, which is the most abundant mineral and results from the corrosion of ferrous metals in seawater. Thus, the leading cause of the corrosion of the metal bottom in saltwater is the chloride ions.
Artifacts containing ferrous and nonferrous materials, will degrade faster in aggressive environments, such as seawater, than in less aggressive ambient conditions[23]. XRD can be used to determine the types of minerals in artifacts and to conserve and inhibit degradation based on the type of metal, which can include copper and its alloys, iron and its alloys, and other metals (including silver, lead, and zinc) [24]. Information about the morphology, elemental composition, and structure of the crystal makes it possible to determine the constituents of the corrosion layer [25].
The reaction mechanism that occurs during underwater metal corrosion is as follows, according to Hamilton [1]:
Reactions that occur on an inert metal surface (cathode)
2H2O + 2e- → H2 + 2(OH)-
Hydroxide ions react with sodium ions in water
Na+ + OH- → NaOH
At the anode, the reaction produces iron ions
Fe2+ + 2e- → Fe
Fe3+ + 3e- → Fe
Fe reacts with chloride ions in salt-containing water (seawater)
Fe2+ + 2CI- → FeCl2
Underwater conditions containing oxygen will continue to form hydroxides during the reaction:
FeCl2 + 2NaOH → Fe(OH)2 + 2NaCl
The hydroxide compound formed in the oxygen-containing solution will experience a secondary reaction, and corrosion deposits will form around the metal surface of the anode:
Fe + 2e- → Fe2+ (Ferrous ion)
Fe2++ 2OH- → Fe(OH)2 (Ferrous hydroxide)
4Fe(OH)2 + O2 → 2H2O + 2Fe2O3 • H2O (Hydrated ferric hydroxide)
Secondary reactions involving ferric ions produce other products during corrosion:
6Fe(OH)2 + O2 → 4H2O + 2Fe3O4 • H2O (Green hydrated magnetite)
Fe3O4 • H2O → H2O + Fe3O4 (Black magnetite)
The XRF analysis of the surface corrosion of the iron cannonball material immersed in water
The results obtained from analyzed the surface corrosion of the iron cannonball material immersed in water using XRF is shown in Table 2.
Table 2
Characterization of surface corrosion using XRF
No
|
Dry active corrosion
|
Wet active corrosion
|
Element
|
Amount (%)
|
Element
|
Amount (%)
|
1
|
Cl
|
66.603
|
Cl
|
64.963
|
2
|
Fe
|
32.105
|
Fe
|
24.730
|
3
|
Ca
|
0.320
|
-
|
-
|
4
|
Mn
|
0.259
|
Mn
|
0.265
|
5
|
Al
|
0.147
|
Al
|
0.095
|
6
|
SiO2
|
0.193
|
SiO2
|
0.195
|
7
|
S
|
0.064
|
S
|
0.085
|
8
|
P
|
0.039
|
P
|
0.051
|
9
|
Cd
|
0.011
|
Cd
|
0.0092
|
10
|
Sb
|
0.0066
|
Sb
|
0.0047
|
11
|
Sn
|
0.0041
|
Sn
|
0.0047
|
Table 2 shows the XRF results of characterizing the corrosion of the iron cannonball underwater. Table 2 shows that the most abundant element in the corrosion products is chlorine. Table 2 shows that the leading causes of the corrosion of the underwater cannonball culture remnants (submerged in seawater) are chloride ions.
SEM characterization of the surface corrosion of the iron cannonball material immersed in water
The SEM results of the characterization of the surface corrosion of the cannonball heritage material is shown in Fig. 7. The cannonball contains hollow cavities, and the iron material is damaged, as shown in Fig. 8.
Passivation/deactivation of the corroded cannonball heritage material
Passivation prevents the corrosion of an object with a particular method. For the passivation method used in this study, the sample is soaked in a solution of 5% sodium carbonate. The pH is maintained between 11-13 using sodium hydroxide. If the pH goes down, then the pH should be increased within the range with a solution of sodium hydroxide, and then, the object should be rinsed with water and subsequently washed using distilled water. The purpose of drying is to see the corrosion development, and if corrosion still happens, then the corrosion process is repeated until corrosion stops. Soaking was conducted for five months, and a mixed solution was replaced after about four days to a week. Before the process, the cleanup crust or rust was not passivated. Layers of the coating would be a natural protector in the meantime. Next, manual cleanup was performed using tools such as brushes, a skavel, needles, a chisel, a hammer and other devices. The cleaning level is adjusted to the level of corrosion, and if the object is still obviously in poor condition, then the object will be made as clean as possible. If the form of the object is already less precise, then cleanup should be performed carefully so that the crust is not released. Not all of the surface should be clean, as to the form of the object should be maintained/found. If corrosion is already very advanced, and shapes are not visible at all, then mechanical cleaning is only performed on the surface.
Alkali solutions, such as sodium hydroxide and potassium hydroxide, will remove rust from iron and steel. Alkali solutions combined with sequestering agents, which are used to hold the dissolved iron in solution, can be very effective, particularly at near-boiling temperatures. Under the Fe3O4 deposit, the following reactions can occur on the substrate:
Fe3O4 + 5H2O → 3Fe(OH)3(s) + H+ + e−
Fe3O4 + 4H2O + OH− + 2e− → 3Fe(OH)3−
Next, cleaning is performed, where the cleaning phase is the core of the conservation activities. Thus, cleaning should be completed to address the conservation problems of the object so that the object can last for a long time. Materials were processed by cleaning with soapy water, using washing fruit extract and kaffir lime water, and rinsing with distilled water. At the end of the flushing and drying stage, flushing is performed to completely remove the chloride solution. After the completion of the drying stage, rinsing is performed. The drying process is carried out by warming in the blazing sun and using a blow dryer.
Stabilizing/coating the iron cannonball underwater heritage material
The coating material used is a microcrystalline wax, and various concentrations of 5, 10, 20, and 50% wax were achieved with turpentine oil solvents. Coating materials, namely, carboxylic monoacids in an ethanol solution [26], acidic solutions from plants [27], and carboxylates, have been applied to iron-based objects by several researchers after the conservation process is completed to inhibit the corrosion process [28]. Microcrystalline wax is used to coat the iron metal that has finished passivation so that the metal is not prone to corrode again. The results of coating using microcrystalline wax can be seen in Fig. 9. Fig. 10. shows the iron cannonball that has been conserved and is ready to be sent to the museum.
Fig. 11 shows some of the different colored iron cannonball materials coated with the microcrystalline solution. From the results of the image, the stabilization or coating of the iron cannonball material was achieved using a 5, 10, 20, and 50% microcrystalline wax solution. The results showed that the most suitable solution did not change the color of the sample, which was achieved by a microcrystalline wax solution with a concentration of 5%. The iron cannonball material is still prone to further corrosion after passivation. Therefore, stabilization/coating must be done as soon as possible. This stabilization stage is the last stage of the passivation process that was completed after all. In this study, the author uses 5% wax microcrystalline with a solvent turpentine oil to coat the iron metal that has finished passivation so that the metal is not prone to further corrosion.
Fig. 11 shows a significant difference between the tangible cultural heritage of the cultural iron cannonball immersed in water before conservation (a) and after conservation (b). Fig. 11 shows that the method of conservation used in this research is very suitable to be used in the conservation of iron material underwater. However, this conservation research is not ideal because the iron cannonball material used in the study existed for far too long on the mainland, this cultural heritage iron material should be directly conserved after the rapture of the deep sea. The results of the analysis and XRF characterization of iron cannonball material objects lingering underwater before and after conservation can be seen in Table 3.
Table 3 shows that after conservation, elemental Cl and some metals have reacted with NaOH. Elemental Cl is the main element causing wet and dry corrosion. After immersion with 5% NaOH, the Cl element reacts with NaOH.
Cl- + NaOH → NaCl(aq) + OH-
The artifact is soaked for one week in 5% NaOH, rinsed with distilled water, and then cleaned physically with a brush; if the corrosion has not stopped, the process of soaking was repeated until corrosion stops. After corrosion stops, the iron cannonball shape was refined. NaOH and the metal react according to the following example:
Zn + 2NaOH → Na2ZnO2 + H2
Al3+ + 3NaOH → Al(OH)3 + 3Na+
Al(OH)3 + NaOH → 2Na+[Al(OH)4]- or
Al + NaOH + H2O → NaAlO2 + H2
Table 3.
XRF characterization of the corroded surface of the iron cannonball material before and after conservation
No
|
Before conservation
|
After conservation
|
Dry active corrosion
|
Wet active corrosion
|
Test 1
|
Test 2
|
Test 3
|
Element
|
Amount (%)
|
Element
|
Amount (%)
|
Element
|
Amount (%)
|
Element
|
Amount (%)
|
Element
|
Amount (%)
|
1
|
Cl
|
66.603
|
Cl
|
64.963
|
Fe
|
98.90
|
Fe
|
98.72
|
Fe
|
99.35
|
2
|
Fe
|
32.105
|
Fe
|
24.730
|
Co
|
0.40
|
Cu
|
0.45
|
Mn
|
0.44
|
3
|
Ca
|
0.320
|
Mn
|
0.265
|
Mn
|
0.31
|
Zn
|
0.37
|
Cu
|
0.12
|
4
|
Mn
|
0.259
|
Al
|
0.095
|
Cu
|
0.20
|
Mn
|
0.29
|
V
|
0.06
|
5
|
Al
|
0.147
|
SiO2
|
0.195
|
Zn
|
0.17
|
Ti
|
0.17
|
Ni
|
0.04
|
6
|
SiO2
|
0.193
|
S
|
0.085
|
-
|
-
|
-
|
-
|
-
|
-
|
7
|
S
|
0.064
|
P
|
0.051
|
-
|
-
|
-
|
-
|
-
|
-
|
8
|
P
|
0.039
|
Cd
|
0.0092
|
-
|
-
|
-
|
-
|
-
|
-
|
9
|
Cd
|
0.011
|
Sb
|
0.0047
|
-
|
-
|
-
|
-
|
-
|
-
|
10
|
Sb
|
0.0066
|
Sn
|
0.0047
|
-
|
-
|
-
|
-
|
-
|
-
|
11
|
Sn
|
0.0041
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
The next step is cleaning with soapy water to remove any residual corrosion products. Soap is an alkaline salt of fatty acids and will thus partially be hydrolyzed by water. Therefore, the soap solution in water is alkaline.
CH3(CH2)16COONa + H2O → CH3(CH2)16COOH + OH- + Na+
The weak base solution obtained from soapy water can help clean surface corrosion products. The iron cannonball material was washed with soapy water and then washed with an aqueous kaffir lime extract (weak acid solution). Citric acid is a type of acid that is nontoxic, nonirritating, and environmentally friendly [29]. Citric acid is also easy to find in citrus-like organic substances, including citrus (kaffir lime) and lemon (citrus lemon). The citric acid content contained in kaffir lime is 45.8 g/L, while the citric acid content contained in lemon is 48.0 g/L [30]. The aqueous kaffir lime extract contains citric acid and ascorbic acid, which are weak acids, and thus removes impurities on the surface of the ball cannon.
The characterization of the iron cannonball material was performed using XRF before and after conservation (Table 3) and showed a significant difference between the data. The object undergoing active wet corrosion and drying before conservation showed the presence of chlorine, which is the result of the corrosion of the iron material, whereas on the object after conservation, the test results showed very significant differences, in which chlorine was no longer detected. From this, it can be concluded that the process of corrosion on the object was lost and stopped.