COPPER ALLOYS IN REAL INDUSTRIAL CONDITIONS: CORROSION AND MICROBIOLOGY

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MICROBIOLOGICALLY INFLUENCED CORROSION (MIC)

The well-known toxicity of cuprous ions towards living organisms does not mean that the copper-based alloys are immune to biological effects on corrosion. The electrochemical nature of the metallic corrosion still remains present in the microbial corrosion. There is an anodic process of metallic dissolution and a complementary cathodic process that is dependent of the metal-biofilm characteristics (pH, aeration, chemical composition…), as the reduction of dissolved oxygen (in aerated environments and neutral pH) or the reduction of water (no-aerated environments). Microorganisms can change the metal/solution interface to induce, accelerate or inhibit the anodic and or cathodic reactions of the corrosion process.
The biological process is illustrated by a microbial colony growing up on the metallic substrate (Fig. 1-10). An anaerobic region is formed under the microbial colony, due to the oxygen consumption by the microbial respiration (in case of aerobic microbes) and another region, where more oxygen reaches the external part of the colony, in contact with the aerated liquid.

EFFECT OF DIFFERENT PARAMETERS ON THE CORROSION BEHAVIOR OF 70CU-30NI ALLOY AND AL BRASS

A copper alloy is the combination of copper with one or more other metals to form a material that can improve the performance of pure copper. Doping with divalent or trivalent cations is an effective way of improving the corrosion resistance of copper.
Copper and copper based alloys can be divided into 3 groups according to the chemical composition [85]: Copper and high copper alloys (copper: Unified Numbering System UNS C10100-C15999; high-copper alloys: UNS C16000-C19999; copper nickels (Cu-Ni-Fe alloys): UNS C70000-C73499 and nickel silvers (Cu-Ni-Zn alloys): UNS C73500-C79999), brasses (Cu-Zn alloys, with or without the addition of Pb and Sn: UNS C20000-C49999) and bronzes (alloys with Sn, P, Al, or Si as the principal alloying element, UNS C50000-C69999).
Copper is the most noble metal in common use. It has excellent resistance to corrosion in the atmosphere and in fresh water. In seawater, copper and copper alloys (particularly those associated with nickel) are widely and successfully used, due to their corrosion resistance, mechanical strength and workability, excellent electrical and thermal conductivity [86] and resistance to macrofouling [87]. In practice, aluminum, zinc, tin, iron, and nickel are often used as alloying elements and reduce noticeably the corrosion rate of copper alloys.

EFFECT OF IRON AND NICKEL

Nickel and iron present in Cu-Ni alloys improve the corrosion and erosion resistance properties of the oxide film. There is beneficial effect of incorporating iron in copper-nickel alloys and it is essential to obtain adequate corrosion resistance by assisting in protective film formation [88, 89]. Copper and its alloys do not form a truly passive corrosion product film. In aqueous environments at ambient temperature, the corrosion product predominantly responsible for protection is cuprous oxide (Cu2O), a p-type semiconductor. It has been established that this is the main component of the protective film formed, in the early stages of growth. The protective properties are enhanced by the incorporation of nickel and iron that leads to a decrease in both ionic and electronic conductivities [89, 90]. The corrosion process can proceed if copper ions and electrons migrate through the Cu2O film. In order to improve the corrosion resistance of the material, the ionic and/or electronic conductivity of the film must be reduced by doping with divalent or trivalent cations [91]. Small additions of iron to copper-nickel alloys are also known to improve their resistance to erosion-corrosion [92] because iron is necessary for the occurrence of nickel enrichment in the corrosion product layer [93].

EFFECT OF OXYGEN CONTENT

The oxygen content of the electrolyte has a significant effect on the corrosion resistance of copper alloys [59]. For instance, at low to moderate oxygen concentrations ([O2] ≤ 6.6 mg/L), the 70Cu-30Ni alloy is more corrosion resistant than the 90Cu-10Ni ones and in saturated seawater; both alloys have similar behavior [59].

EFFECT OF POLLUTED SEAWATER

Copper-nickel alloys corrode at increased rates in polluted waters (compared to clean waters), particularly when sulfides or other sulfur compounds are present [48, 94, 102]. Sulfides form a black corrosion product, which is less adherent and protective than the normal oxide film.
Gudas and Hack reported that 0.05 ppm of sulfide or more was necessary to cause increased corrosion of 70Cu-30Ni alloy, and even 0.01 ppm of sulfide, during one day, caused accelerated attack of 90Cu-10Ni alloy [103]. This is in agreement with the work of Alhajji and Reda who noted that sulfide was very corrosive towards the alloys with low nickel content [104].
The corrosion of Cu-Ni alloys, in flowing (1.6 m/s) seawater containing sulfides, polysulfides, or sulfur, was investigated by Anderson and Badia [105] and MacDonald et al. [62]. Cuprous sulphide forms as the principal corrosion product causing damage to the protective film on the metal surface. The importance of proper protective film formation on tubes and pipes must be emphasized. It is stated that if during the early life of condenser tubes clean seawater passes through, good protective films will form which are likely to withstand most adverse conditions. The ideal situation, whether in a ship or a power plant, is to re-circulate aerated, clean seawater from initial start up to a time sufficient enough to form a good protective film. If, however, polluted waters are encountered during the early life, the films formed on the condenser tubes will likely not be fully protective and the risk of premature failure will be considerably increased [102].

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Table of contents :

CHAPTER 1 – SCIENTIFIC BACKGROUND
1.1 Cooling water systems in power supply facilities
1.1.1 Types of condensers
1.1.2 Types of cooling waters
1.1.3 Constituent materials of cooling water systems
1.1.4 Cooling water systems operating problems
1.1.5 Cooling circuits tubes cleaning methods
1.1.6 Chlorination
1.2 Corrosion of copper and its alloys in aqueous environments
1.2.1 Galvanic corrosion
1.2.2 Pitting corrosion
1.2.3 Dealloying
1.2.4 Ammonia attack
1.2.5 Sulfide attack
1.2.6 Erosion-corrosion
1.2.7 Microbiologically influenced corrosion (MIC)
1.3 Effect of different parameters on the corrosion behavior of 70Cu-30Ni alloy and Al brass
1.3.1 Effect of iron and nickel
1.3.2 Effect of temperature
1.3.4 Effect of pH
1.3.4 Effect of oxygen content
1.3.5 Effect of polluted seawater
1.3.6 Effect of water velocity
1.3.7 Effect of seawater treatments
1.3.8 Effect of suspended particles and mud
1.3.9 Effect of biomolecules
1.4 Aim of this thesis and research strategy
1.5 Thesis outline
CHAPTER 2 – MATERIALS AND METHODS
2.1 Studied metallic materials and electrolytes
2.1.1 Metallic materials
2.1.1.1 Field experiments – electrodes and surface preparation
2.1.1.2 Laboratory experiments – electrodes and surface preparation
2.1.2 Electrolytes
2.1.2.1 Field experiments
 Natural seawater (NSW)
 Treated natural seawater (TNSW)
2.1.2.2 Laboratory experiments
 Filtered natural seawater (FNSW)
 Artificial seawater (ASW)
 Artificial seawater + BSA (ASW+BSA)
2.2 Experimental methods
2.2.1 Field experiments
2.2.1.1 On-line measurements
 Electrochemical cell
 Corrosion potential (Ecorr) vs immersion time
 Linear Polarization Resistance (LPR)
2.2.1.2 Off-line measurements
M.L.CARVALHO
 Weight loss measurements
 Genetic studies
2.2.2 Laboratory experiments
2.2.2.1 Electrochemical measurements
 Electrochemical cells
 Corrosion potential (Ecorr) vs immersion time
 Polarization curves
 Levich and Koutecky-Levich methods
 Electrochemical Impedance Spectroscopy (EIS)
2.2.2.2 Surface analysis
 X-ray Photoelectron Spectroscopy (XPS)
 Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS)
CHAPTER 3 – COPPER ALLOYS IN REAL INDUSTRIAL CONDITIONS: CORROSION AND MICROBIOLOGY
3.1 Power plants characteristics
3.1.1 Tests on the Tyrrhenian coast
3.1.2 Tests on the Adriatic coast
3.2 Electrochemical tools
3.2.1 Corrosion rate
3.2.2 Biofilm growth
3.2.3 Chlorination treatments
3.2.4 Other physico-chemical parameters
3.3 Results of the monitoring campaign
3.3.1 70Cu-30Ni alloy – Online measurements
3.3.2 Al brass – Online measurements
3.3.3 70Cu-30Ni alloy and Al brass – Off-line measurements
3.3.3.1 Visual observations
3.3.3.2 Weight loss measurements
3.3.3.3 Microbiological analyses
3.4 Conclusions
CHAPTER 4 – EFFECT OF SOLUTION AND BIOMOLECULE CONCENTRATION
4.1 Results
4.1.1 Electrochemical measurements
4.1.1.1 Corrosion potential (Ecorr) vs time
4.1.1.2 Cathodic and anodic polarization curves
4.1.1.3 Electrochemical Impedance Spectroscopy
4.1.2 Surface analysis
4.1.2.1 Equations necessary for XPS data processing
4.1.2.2 Results
4.2 Discussion
4.2.1 Surface layers models (combined XPS and ToF-SIMS)
4.2.2 Composition of the organic layers (XPS)
4.2.3 Corrosion mechanism
4.2.4 Impedance model for 70Cu-30Ni and EIS data fitting
4.3 Conclusions
CHAPTER 5 – EFFECT OF HYDRODYNAMICS
5.1 Static conditions vs under flow and stirring – Results
5.1.1 Electrochemical measurements
5.1.2 Surface analysis
5.1.2.1 Surface layers models (combined XPS and ToF-SIMS)
5.2 Electrochemical measurements using a RRE
5.2.1 Theory for a Rotating Ring Electrode
5.2.2 Results
5.2.2.1 70Cu-30Ni
5.2.2.1.1 Corrosion potential (Ecorr) vs time
5.2.2.1.2 Cathodic polarization curves
5.2.2.1.3 Levich and Koutecky-Levich curves
5.2.2.1.4 Anodic polarization curves
5.2.2.1.5 Electrochemical Impedance Spectroscopy
5.2.2.1.6 EIS data fitting
5.2.2.2 Al brass
5.2.2.2.1 Corrosion potential (Ecorr) vs time
5.2.2.2.2 Cathodic and anodic polarization curves
5.2.2.2.3 Electrochemical Impedance Spectroscopy
5.2.2.2.4 Impedance model for Al brass and EIS data fitting
5.3 Conclusions
CHAPTER 6 – EFFECT OF PH
6.1 70Cu-30Ni
6.1.1 Electrochemical measurements
6.1.1.1 Corrosion potential (Ecorr) vs time
6.1.1.2 Cathodic polarization curves
6.1.1.3 Anodic polarization curves
6.1.1.4 Electrochemical Impedance Spectroscopy
6.1.1.5 EIS data fitting
6.1.2 Surface analysis
6.1.2.1 Results
6.1.2.2 Surface layers models (combined XPS and ToF-SIMS)
6.1.2.3 Composition of the organic layers (XPS)
6.2 Al brass
6.2.1 Electrochemical measurements
6.2.1.1 Corrosion potential (Ecorr) vs time
6.2.1.2 Cathodic polarization curves
6.2.1.3 Anodic polarization curves
6.2.1.4 Electrochemical Impedance Spectroscopy
6.2.1.5 EIS data fitting
6.2.2 Surface analysis
6.2.2.1 Introduction
6.2.2.2 Results
6.2.2.3 Surface layers models (combined XPS and ToF-SIMS)
6.3 Conclusions
GENERAL CONCLUSIONS
REFERENCES
ANNEX A – CHAPTER 4
ANNEX B – CHAPTER 5
LIST OF FIGURES
LIST OF TABLES

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