The Influence of Archaeometallurgical Copper Alloy Castings Microstructure towards Corrosion Evolution in Various Corrosive Media
Abstract
:1. Introduction
2. Materials and Methods
- (OCP+ 0.6 V, scan rate 1 mV/s): OM observations on the corroded electrode surfaces were conducted in some representative areas of TB electrochemical patinas. Patina fragments were collected from all three electrodes to perform morphological examination by SEM and chemical analyses by FTIR and EDS. Patina fractions were detached using a carbon adhesive tape to examine both internal and external surfaces by SEM-EDS. The EDS data (elemental analyses) were processed to serve the investigation of the alloy/patina and patina/electrolyte interface reactions. Similar analyses were also conducted on the metal substrate (areas under detached patinas). Finally, powder micro-samples were mechanically scraped from the three corroded electrode surfaces and were homogenized in order to produce KBr pellets. The FTIR measurements were conducted by an JASCO FT/IR-4200 spectrometer (JASCO International Co. Ltd., Tokyo, Japan) with a TGS detector. All spectra were recorded at a scan range of 4000–400 cm−1, with accumulation set to 32 and resolution to 4.0 cm−1, and underwent baseline correction, smoothing and CO2 peak reduction through Spectra Manager software.
- (OCP+ 1.5 V, scan rate 0.25 mV/s): The same methodology was applied for the characterization of TB and ZB electrochemical patinas after the end of anodic polarization sweeps in 0.1 mol/L NaCl. These systematic chemical and morphological analyses were employed in order to study the elemental distribution as a result of the alloy dissolution and the precipitation of corrosion compounds at the metal/patina and patina/electrolyte interfaces. In the case of ZB patina, a brief characterization of corrosion patterns by OM and SEM-EDS has been published by the authors in a previous work [29], together with time-lapse photographic documentation of the electrochemical reactions during the sweep and comments on the electrochemical curves.
3. Results
3.1. Metallographic Characterization of Reference Archaeometallurgical Alloys
3.2. Accelerated Corrosion in Total Immersion Conditions/Anodic Polarization in Various Electrolytes
3.2.1. 1st Experimental Section—Characterization of Electrochemical Patinas in Three Synthetic Electrolytes
3.2.2. 2nd Experimental Section—Characterization of Electrochemical Patinas in 0.1 mol/L NaCl
- TB Casting
- ZB Casting
4. Discussion
- Macro-segregation affects the oxidation and dealloying rates at a local scale, according to the geometry of the solidification front. Disc-shaped specimens cut from rod castings exhibit lower Sn concentration and smaller grain sizes at the periphery and higher Sn and larger grains at the specimen centre. The corrosion attack initiates from the Cu-enriched edge and develops towards the centre.
- The differential chemical composition of dendritic microstructure inside the grains is responsible for the action of local micro-galvanic cells where the formation of different corrosion compounds is favoured and variant dealloying rates occur. The establishment of anodic and cathodic sites means that different electrochemical actions occur locally. As a result, the nature of corrosion products is also site-specific.
- In the studied α-bronze and α-brass corrosion systems, the electrochemical attack initiates from the dendrites which act as anodes, while the interdendritic areas are the cathodic sites. All initial corrosion products are epitaxially grown on the given microstructure and tend to dissolve faster.
- The TB casting—exhibiting a cored dendritic microstructure—has undergone more severe corrosion in the atmospheric pollutants mixture compared to the soil filtrate and the 0.6 mol/L NaCl, after anodic polarization (OCP+ 0.6 V) in these electrolytes. The lead phases were selectively attacked by sulphates and nitrates.
- The main corrosion products identified by FTIR in the electrochemical patinas (PRSF, AAPM, 0.6 mol/L NaCl) of TB casting were basic Cu(II) hydroxychlorides, basic Cu(II) hydroxycarbonates and amorphous Cu and Sn oxides. In AAPM, some additional bands corresponding to Cu(II) hydroxysulphate compounds were detected.
- In the first experimental section, it was evidenced that, at moderately anodic potential ranges (OCP+ 0.6 V), the external patina surface still exhibits local corrosion features evolved on the pre-existing dendritic structure chemical inhomogeneity. However, the electrochemical and chemical reactions at the patina/electrolyte interface tend to eliminate or counter-balance the inhomogeneous distribution of alloying elements. Thus, at high anodic potentials (OCP+ 1.5 V), the external surface of TB and ZB electrochemical patinas exhibits uniform chemical composition. This was confirmed by the characterization of patinas, presented in the second experimental section.
- The stratification of electrochemical patinas produced on TB and ZB castings by slow anodic polarization in 0.1 mol/L NaCl reveals a decuprification profile for TB and synchronous dezincification and decuprification for ZB.
- The ternary bronze casting (TB) exhibits selective oxidation of the Cu-enriched dendrites and higher decuprification rate in these areas. The corroded interdendritic areas in ternary bronze remain Sn-enriched in respect to the average of the particular surface, and the corrosion compounds exhibit a Cl/O atomic ratio of 0.75, lower than the value calculated for the dendrite network (1.33), which is mainly attacked by chlorides. The Cl/O ratio of the external patina surface is determined as 2.80.
- The quaternary brass casting (ZB) exhibits faster dealloying processes under the same conditions. Selective attack of Zn initiates from the dendritic network. Dezincification progresses faster at these areas, as indicated by Zn elimination from the internal corrosion layers, and is further assisted by the higher solubility of the Zn chloride species. Decuprification rate is uniform in all segregated areas. The two processes at the alloy/patina interface leave behind a metal surface, where α-dendrites are enriched in Sn compared to the alloy matrix. Both patina interfaces (with metal and electrolyte) exhibit a higher Cl/O atomic ratio compared to the Cu-Sn-Pb alloy—the calculated values are 1.59 and 6.64, respectively.
- On the external patina surfaces of both TB and ZB castings, after slow anodic polarization (OCP+ 1.5 V) in 0.1 mol/L NaCl, the uneven elemental distribution is balanced by dissolution redox reactions and redeposition of Cu complexes.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
References
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Cl− (mg/g) | SO42− (mg/g) | HCO3− (mg/g) | TOC (mg/g) | Mg (mg/g) | Fe (mg/g) | Ca (mg/g) |
---|---|---|---|---|---|---|
11.5 | 25.0 | 0.5 | 1.2 | 32.0 | 4.9 | 98.0 |
SO42− (mol/L) | NO3− (mol/L) | Cl− (mol/L) | HCO3− (mol/L) |
---|---|---|---|
0.050 | 0.025 | 0.008 | 5 ·10−5 |
Element | Dendritic Network | Interdendritic Areas | Leaded Areas |
---|---|---|---|
O | 76.0 | 71.5 | 73.2 |
Cl | 22.8 | 27.0 | 14.0 |
S | 1.2 | 1.5 | 8.0 |
N | - | - | 4.8 |
Corrosive Medium | Type of Vibration | Bands (cm−1) | Attribution | Reference |
---|---|---|---|---|
PRSF | OH stretching | 3522 (w), 3447 (s), 3355, 3339, 3327 (triple broad peak) | Cu2(OH)3Cl—intermediate of botallackite and atacamite polymorphs | [30,31,32] |
AAPM | 3522 (w), 3450 (s), 3355, 3340 (triple broad peak) | |||
0.6 mol/L NaCl | 3522 (w), 3447 (s), 3352, 3347, 3327 (triple broad peak) | |||
all electrolytes | in-plane OH deformation of H2O | many weak bands within 1620–1660 range | Sn oxyhydroxides | [33] |
all electrolytes | weak overtones of Sn-O-Sn and Sn-O-H | 1558, 1541, 1521, 1507, 1472, 1457, 1409 | Sn oxyhydroxides | [34] |
PRSF | 1385 (strong) | Cu2(OH)2CO3—malachite | [35,36] | |
AAPM | 1385 (weak) | |||
0.6 mol/L NaCl | 1385 (weak) | |||
AAPM | internal vibration of SO42− | 1124 | Cu4(OH)6SO4—brochantite or posnjakite or Sn oxyhydroxides | [34,35] |
0.6 mol/L NaCl | weak overtones of Sn-O-Sn and Sn-O-H | 1113 | Sn oxyhydroxides | |
PRSF | OH deformation | 988, 952, 926, 919, 897, 849 | Cu2(OH)3Cl—atacamite | [30,31,32,35] |
AAPM | 988 *, 952, 924, 917, 897, 849 | Cu2(OH)3Cl—atacamite * also Cu4(OH)6SO4—brochantite (Cu-OH bending) | [30,31,32,35] | |
0.6 mol/L NaCl | 988, 952, 922, 897, | Cu2(OH)3Cl—atacamite | [30,31,32] | |
All electrolytes | 834 (sh), 823 (sh) | Cu2(OH)2CO3—malachite | [35] | |
all electrolytes | Sn-O or Cu-O vibrations | 600 (broad) | nanocrystalline or amorphous SnO2 or Cu2O | [36] |
PRSF | Cu-O and Cu-OH | 526 (sh), 518, 506 | intermediate of atacamite and botallackite, malachite | [31,35] |
AAPM | 526, 515, 508 | |||
0.6 mol/L NaCl | 526 (sh), 518, 508 | |||
PRSF | Cu-O stretching | 492, 482 | Cu2(OH)2CO3—malachite and Cu4(OH)6SO4 —brochantite | [35] |
AAPM | 489, 482 | |||
PRSF | Cu-O stretching | 474 | Cu2(OH)3Cl—atacamite | [31] |
AAPM | 473 | |||
0.6 mol/L NaCl | 474 | |||
PRSF | Cu-O stretching | 459, 450, 444, 435 | Cu2(OH)3Cl—botallackite and atacamite | [30,31] |
AAPM | 458, 444 | |||
0.6 mol/L NaCl | 458, 444, 434 | |||
PRSF | Cu-O stretching or internal vibration of SO42− | 419 | Cu2(OH)3Cl—botallackite | [31,35] |
AAPM | 420 | Cu2(OH)3Cl—botallackite or Cu4(OH)6SO4—brochantite | ||
0.6 mol/L NaCl | 419 | Cu2(OH)3Cl—botallackite |
Element | TB | ZB | |||
---|---|---|---|---|---|
Interface with Alloy | Interface with Electrolyte | Interface with Alloy | Interface with Electrolyte | ||
Dendritic Network | Interdendritic Areas | ||||
O | 43.0 | 57.1 | 26.3 | 38.6 | 13.1 |
Cl | 57.0 | 42.9 | 73.7 | 61.4 | 86.9 |
Cl/O | 1.33 | 0.75 | 2.80 | 1.59 | 6.64 |
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Papadopoulou, O.; Vassiliou, P. The Influence of Archaeometallurgical Copper Alloy Castings Microstructure towards Corrosion Evolution in Various Corrosive Media. Corros. Mater. Degrad. 2021, 2, 227-247. https://0-doi-org.brum.beds.ac.uk/10.3390/cmd2020013
Papadopoulou O, Vassiliou P. The Influence of Archaeometallurgical Copper Alloy Castings Microstructure towards Corrosion Evolution in Various Corrosive Media. Corrosion and Materials Degradation. 2021; 2(2):227-247. https://0-doi-org.brum.beds.ac.uk/10.3390/cmd2020013
Chicago/Turabian StylePapadopoulou, Olga, and Panayota Vassiliou. 2021. "The Influence of Archaeometallurgical Copper Alloy Castings Microstructure towards Corrosion Evolution in Various Corrosive Media" Corrosion and Materials Degradation 2, no. 2: 227-247. https://0-doi-org.brum.beds.ac.uk/10.3390/cmd2020013