الفهرس 
Introduction, Literature Survey and Aim of the workOnly 14 pages are availabe for public view
 |  | from | 249 |  |  |
| | | from | 249 | | |
Abstract
This thesis can be summarized as following:
Chapter I
This chapter included the introduction concerning background on refrigeration absorption systems and corrosion of Cu alloys. It covered the literature on corrosion problems in refrigeration absorption systems, effect of some cations on the corrosion of Cu-alloys, the effect of corrosion product and de-alloying in aqueous solutions. In addition, this chapter covered the aim of the thesis.
Chapter II
This chapter showed the materials and experimental techniques set up of the study. Measurements were made on Al-bronze (Cu/7 Al), Brass (Cu/30 Zn), Cupronickel (Cu/10 Ni and Cu/30 Ni) in LiBr solutions.
The potentiodynamic cyclic polarization, chronoamperemetry and electrochemical techniques, which were complemented by different methods for Scanning Electron Microscope (SEM) equipped with Energy Dispersive X-ray (EDX) and electrolytic solution analysis measurements by Atomic Absorption Spectrometry (AAS).
Chapter III
This chapter described four important topics
1- The Corrosion Behaviour of Cu-alloys in Different Concentrations of LiBr Solutions up to 9 M
This part devoted the effect of different concentrations of LiBr solution up to 9 M on Cu alloys. Cyclic potentiodynamic polarization of different studied alloys showed that the corrosion potential shifted to more negative values as the LiBr concentrations increased. At ≤ 0.5 M LiBr, two anodic peaks (P1, P2) appeared. P1 formed due to oxidation of Cu metal to CuBr and or Cu2O while P2 was as a result of oxidation of Cu+ to CuO and or Cu (OH)2.
By increasing concentration of LiBr ≥ 1 M, as a result of the deleterious effect of Br-, P2 disappeared due to lose its passivity through soluble di-bromo cuprate complex (CuBr2-).
Different features obtained:
The highest values for Ecorr. and ICorr. recorded in case of Cu/7 Al than the other alloys. On the contrast, passivity rang, breakdown potential and hysteresis loop area for Cu/7 Al appeared at lower values.
For Cu/10 Ni at 0.5 M LiBr, two breakdown potentials, the first one appeared at 700 mV and the second one was observed at more anodic potential while Cu/30 Ni, pitting corrosion was manifested at 2 and 4 M LiBr only.
The breakdown potential, the passive current and the hysteresis loop area began to appear at 10-1 M which were disappeared after 1 M except for Cu/30 Ni it recorded at 2 M and absence at 6 M.
The above data is consistent with the data of Pourbaix diagram.
Potentiostatic Polarization and Surface Examination in 5x10-1 M before and after breakdown potential in all alloys
Cu/7 Al
Chronoamperometric curves of Cu/7 Al at different potentials of 300 and 600 mV showed that fluctuations and oscillations in current at 300 mV increased by increasing potential than the other alloys. This changes that took place on the surface due to the simultaneous formation film of CuO, Cu(OH)2 and Al2O3
Examination of the sample surface after experiment at 300 mV recorded that there are large number of initiating pits which became propagated at 600 mV in addition to the presence of white layer which was mainly Al2O3 beside the presence of Cu oxide. The EDX analysis of the film registrant the enrichment of Al on the surface of alloy.
Cu/30 Zn
In case of Cu/30 Zn, in 5x10-1 M LiBr solution at constant potential of 300 mV, chronoamperometry indicated the formation of partial passive film as a result of dissolution through diffusion. Dissolution rate increased after breakdown potential at 900 mV.
Surface examination of Cu/30 Zn after the end of experiment the chronoamperometic experiment performed at constant potential of 300 mV indicated a quite homogenous film on the whole surface. The film was formed as white porous which mainly was CuBr. The analysis of the film by EDX pointed out higher concentration percentage from Cu and Br rather than oxygen.
By examining the Cu/30 Zn surface after current time at 900 mV showed that the major parts of the surface suffered from pitting initiation and some of propagation which were blocked by the corrosion product. EDX analysis exhibited high percentage of Br inside pits than outside pits while the surface outside pits was mainly composed of Cu oxide.
The surface of the alloy before and after breakdown potential suffered from dezincification as it was supported by solution analysis which showed that the increase in Zn content in solution confirmed the dezincification of the surface. Both surface examination and solution analysis showed that Cu and Zn were present from the initial stage of polarization and Zn content on the surface was lower than that present in the bulk alloy while in solution Zn is higher than in the substrate. This indicated that a simultaneous dissolution was involved in the dezincification process.
Cu/10 Ni
The current density values recorded before breakdown potential was lower than that of after breakdown potential. The surface examination of Cu/10 Ni alloy after applying a constant potential of 300 mV showed that the surface was covered with a non-porous and homogenous protective copper oxide film. On the other hand, the surface after breakdown potential was covered with continuous adherent oxide film accompanied with some initiation and propagation of pitting. The film formed was CuO and/or Cu (OH)2. In both chronoamperometric measurement, at applied constant potentials of 300 and 900 mV, the surface suffered from denickelification.
Cu/30 Ni
A partial passive current was observed at higher potential which occurred as a result of a competition between the formation of the partial passive and the dissolution of the alloy. The alloy suffered from higher dissolution at 5x10-1 M through the non-protective film formed where the surface suffered from the denickelification. On the other hand, Cu/30 Ni showed very small values of corrosion current (Icorr.) when compared to Cu/10 Ni and other alloys.
from the previous studies, it’s necessary to mention that Cu/30 Ni was the most resistance alloy than the other studied alloys. So that the study extended to Cu/30 Ni alloy only.
2- Effect of Different Types of Cations and Soluble Corrosion Product on the Dissolution of Cu/30 Ni in LiBr Solution
As previously indicated, at higher concentrations of 6 and 9 M LiBr, the formation of CuBr was preferred rather than Cu2O formation. Therefore, the dissolution of Cu passed through CuBr2- soluble complex which confirmed the general dissolution. In the industrial field, Cu alloys behaved somehow differently and corroded mainly as localized pitting attack.
Therefore, it was necessary to study the effect of cations such as Cu, Ni, Zn and Al on the dissolution of Cu/30 Ni at higher concentrations of LiBr solutions from (4-9 M).
Effect of different types of cations
The potentiodynamic cyclic polarization of Cu/30 Ni in 4 M LiBr without any addition observed two types of pitting. One of them was observed at lower anodic potential while the second type was appeared at higher anodic potential. Moreover, two anodic steps appeared at -300 and -200 mV.
The addition of different concentrations of cations such as Cu, Al, Zn and Ni to 4 M LiBr didn’t show any types of local attack. The addition of different concentrations of Cu++ to 4 M LiBr resulted in the disappearance of pitting corrosion. On the other hand, the additions of Cu++ decreased the corrosion potential depended on the concentration of Cu++ addition. While, Ni additions showed two steps when added at concentration of 0 and 0.15 ppm. Further increase of Ni concentration, resulted in disappearance of the second step.
By the addition of 0.4 ppm Zn++, one step appeared only at -170 mV. The second step shifted to -150 mV and became clearer by addition of 2 ppm Zn++, ECorr. was shifted to more negative value by increasing the concentration of cation added. Upon the addition of Al+++, one step was observed.
On the other hand, IMax. was decreased to less negative value in presence of Cu++ and Ni++ while it was increased in case of Zn++ and Al+++.
Effect of Potentiodynamic Cyclic Polarization on the dissolution of Cu/30 Ni in 4 M LiBr
By increasing the number of cycles polarization, ECorr. was shifted to more noble direction. The working electrode area decreased with increasing the number of cycles. This referred to the influence of LiBr concentrations.
Effect of soluble corrosion product
1 Effect of soluble corrosion product of Cu/30 Ni on the corrosion of Cu/30 Ni
As increasing the concentration of LiBr from 2 to 9 M, ECorr. shifted toward more negative value, IPit. was increased to more anodic value except at 9 M LiBr where it was reduced. By increasing LiBr concentration, the passive length and hysteresis loop area increased. This was attributed to conversion of formed film to more passive character where pitting attack occurred. With exception of 9 M LiBr, the more aggressive of the solution made general dissolution as well as pitting attack. The explanation was due to the formation of a doped Cu2O with Ni to form a more passive film. In addition, CuBr2- was formed, which confirmed the general dissolution through the autocatalytic reaction.
2 Effect of soluble corrosion product of different Cu-alloys in 4 M LiBr on the corrosion of Cu/30 Ni
Pitting corrosion occurred for Cu/30 Ni in the solution of 4 M LiBr containing the corrosion product of different alloys. Maximum hysteresis loop area obtained in corrosion product of Cu/7 Al and the minimum area appeared in solution Cu/30 Zn.
In the first cycle of Cu/30 Ni in 4 M LiBr solution, general dissolution took place, while the second cycle, different corrosion product of the different alloys studied in the same concentration of LiBr. Pitting corrosion occurred at low potential as a result of the thickening of the oxide film of Cu2O where the dissolution of the Cu-alloys occurred at this higher concentration where CuBr2- complex is formed accordingly:
There were two types of Cu2O, one of them is the inner doped Cu2O with Ni while the second was due to the deposited Cu2O as a result of hydrolysis of CuBr2- as shown in previous equation.
In the industrial field, different Cu-alloys were used as materials for pipes and heat exchangers tubes in absorption refrigeration systems where LiBr heavy brine was used as refrigerant. The above interpretation could explain why in the industrial field pitting corrosion occurred. While in our studies, general dissolution took place in the absence of corrosion products. Consequently, CuBr2- complex was responsible for the pitting corrosion in Cu/30 Ni. CuBr2- was present as a result of the dissolution of any Cu-alloys in a circulating system.
3- The behaviour of Cu/30 Ni alloy in LiBr solutions
1- Potentiodynamic cyclic polarization curves of Cu/30 Ni alloy in 2 M LiBr exhibited an anodic passivation current which observed at low noble anodic potential of ≈ -200 mV as a result of the formation of a doped Cu2O. On the other hand, Ni from alloy segregated to the formed Cu2O barrier layer and incorporated into cationic vacancies leaded to increase its corrosion resistance and the surface became enrichment of Ni.
2- At higher potential ≥ 300 mV, after EMax. a partially passive film of Cu2(OH)3Br formed and the surface contained more Cu than Ni (deneckilfication).
3- Atomic absorption solution analysis confirmed the surface analysis where Cu on the surface was observed at higher at. % showed a corresponding lower ratio in solution. Similar results observed for Ni and vice versa.
4- Two types of pitting were observed only at 2 and 4 M LiBr. Other concentrations of lower or higher LiBr only showed the first type of pitting were observed at higher anodic current and general dissolution through the partially soluble Cu2(OH)3Br was took place.
4- Effect of Inhibition of Some Amino Acids on the Corrosion Behaviour of Cu/30 Ni in LiBr Solutions
The potentiodynamic polarization curves in the presence of different amino acids L-Lysine mono hydrochloride (L-Lys.), L-Aspartic acid (L-Asp.), L-Methionine (L-Met.), L-Cysteine (L-Cys.) were investigated as corrosion inhibitors showed that ECorr. was shifted negatively. This happened as a result of a decrease in the rate of cathodic reaction which in return attributed to the adsorbed layer of the amino acids on the alloy surface which behaved as a cathodic inhibitor.
By increasing the concentration of amino acids, the slope of the passive region was observed due to competition between the formation of the protective passive region of doped Cu2O and the adsorption of the amino acid on the surface. Also, the corrosion current decreased and the inhibition efficiency increased while a maximum efficiency (η) of 76% was observed in case of 10-1 M L-Cys. addition. In this case, this beahviour may be due to that the inhibitor was not covering the surface, but only occupied the electrochemically active sites which reduced the cathodic and/or the anodic reactions. The corrosion rate decreased by increasing the number of the electrochemically active sites blocked by the adsorbed inhibitor.
The highest inhibition efficiency of L-Cys. was 76% compared to L-Lys., L-Asp. and L-Met. of 64.9, 72.40, 70.8% respectively at a conc. of 10-1 M was due to the presence of S-H and NH2 groups. This indicated that the adsorption was mostly happened through the sulfur in SH group.
Effect of Iodide ions
The effect of the addition of 10-2 M KI on the dissolution of Cu/30 Ni in the presence of 10-5 M of each of amino acids was studied. The order of inhibition efficiency of 10-5 M amino acid in the presence of 10-2 M KI with L-lys., L- Met., L-Asp., L-Cys. was 94, 91, 80 and 60 % of L-Lys., L-Asp., L-Met. and L-Cys. respectively.
The strong chemisorption of iodide ion at the surface of the metal was responsible for the synergistic effect of iodide ion and the amino acid through the combination of this anion and the cations of the inhibitor. The adsorbed anions, after stabilized with the cation of the amino acid, led to the increase of the surface coverage and accordingly increase of the inhibition efficiency.
Chronoamperometric Measurement
Current-time curve of Cu/30 Ni alloy in 2 M LiBr with 10-5 and 10-2 M of L-Lys., L-Asp., L-Met. and L-Cys. was studied which showed the following:
Higher current density was recorded in presence of 10-5 and 10-2 M of L-Lys. and L-Met. in comparison with blank (2 M LiBr).
In case of 10-2 M L-Asp., the observed current density was generally similar to that of blank. This meant that 10-5 M concentration was not enough to cover and protect the alloy surface when compared to that of 10-2 M of the inhibitor.
While 10-5 and 10-2 M L-Cys., the current densities recorded in both cases are slightly shifted to less negative values than the blank. This indicated that L-Cys. had an almost negligible inhibiting effect.
The addition of 10-5 M for both of L-Lys. and L-Met. in the presence of 10-2 M KI retarded the dissolution of the alloy as a result of the synergistic effect. On the other hand, the addition of 10-2 M of KI to 10-5 M of both of L-Asp. and L-Cys. accelerated the corrosion rate as a result of the current densities increase.
There was a difference between the effects of addition of 10-2 M KI to 10-5 M L-Met. and to 10-5M L-Cys. where the first acted as an inhibitor and the second was accelerator, although both contain sulphur, which is believed to be responsible for the strong adsorption. This happened where S-H group of L-Cys. was stronger adsorption than S-CH3 group of L-Met.
The Surface Analysis
Examination the surface of the alloy after equilibration in solution of 10-5 and 10-2 M of each amino acid studied in the absence and in the presence of 10-2 M of KI.
In case of 10-5 M L-Lys, SEM indicated that an initiation and propagation of some pits formed which was responsible for the increase in the current density by time. The atomic concentration ratio confirmed the adsorption of L-Lys. on the alloy surface. On the other hand, enrichment of Ni on the surface. While AAS completed and supported the picture with surface analysis.
After the addition of 10-2 M KI to 10-5 M L-Lys. in the presence of 2 M LiBr, examination of the alloy surface showed a homogenous distribution of the film formed. This confirmed that iodide ion of (-ve) charge adsorbed on the alloy surface which acted on the adsorption of the zwitter ion of amino acid by attraction to produce a film of iodide amino acid which extended to cover the whole surface. The EDX analysis confirmed the presence of I beside C, N and O which was the elements present in L-Lys..
SEM indicated that the film formed after addition of 10-5M L-Asp. showed different initiation and propagation of pits. This occurred as a result of the competition between the oxide film formation and the adsorption of the amino acid which tended to partial adsorption of amino acid. The film formed as a result of adsorption of amino acid was not protective enough to prevent the formation of pits in this aggressive medium (2 M LiBr). EDX analysis of the alloy surface at the above condition showed that the presence of C, O, N which confirmed the adsorption of L-Asp. on the alloy surface.
After the addition of 10-2 M KI to 10-5 M L-Asp. in the presence of 2 M LiBr exhibited that there was some pitting initiation along a weak points on the surface where EDX analysis showed that most of the surface are covered with KI amino acid film. Also, EDX analysis confirmed the adsorption of L-Asp. and I where a peak of C, O, N and I are present among the constituent of the alloy surface.
Addition of 10-5 M L-Met. to 2 M LiBr, the current density increased which represented that L-Met. acted as an accelerator and not as an inhibitor. SEM of the surface at the end of experiment, the surface shows the presence of some pits at the weak point which spreaded over the whole surface. EDX analysis confirmed the adsorption of L-Met. on the surface where C, O, N and S are present with the constituent of the alloy surface.
On the other hand, the SEM of the surface after addition 10-2 M KI to 10-5 M L-Met. in 2 M LiBr showed that the surface was completely covered by iodide Met. film which prevent any attack. This happened where iodide ion adsorbed on the surface by attraction force with cations on the surface. EDX analysis on the surface showed the presence of C, N, O, S and I.
SEM examination of the surface of the alloy in presence of L-Cys. showed that there was no any attack which confirmed the current–time curve. The EDX analysis showed the presence of C, N, O and S which confirmed the adsorption of L-Cys. on the surface. The SEM of the surface after the addition of 10-2 M KI to 2 M LiBr + 10-5 M L-Cys., the film formed showed some pitting which is propagate and spread over the surface. These pitting were responsible for the increasing of the current in comparison with the KI free curve.