Shallow cryogenic treatment: effect on the corrosion resistance and hardness properties of AA5083-H111 alloy in chloride-ions enriched medium

The influence of cryogenic and cryogenic time (10, 24, 36, 48, and 72 h) on the hardness, surface roughness, and corrosion resistance properties of AA5083-H111 alloy in 3.5 wt.% NaCl solution have been investigated. The hardness property was analysed via the Brinell hardness measurement, the corrosion resistance property was measured electrochemically, while the morphological studies were undertaken using the SEM (scanning electron microscopy) and atomic force microscopy (AFM). The results disclose that the shallow cryogenic treatment at −80°C improves the alloy’s hardness, surface roughness, and the corrosion resistance. The best cryogenic treatment time is 24 h. Cryogenic treatment for 24 h increases the alloy’s hardness from 71.3 HB to 74.90 HB, reduces the average surface roughness from 534.000 nm to 105.634 nm, and increases the total charge transfer resistance from 18139 Ω cm2 to 26230 Ω cm2. The improvement is linked to settling of fine particles on the alloy surface. The SEM results support these claims.


Introduction
There is a fast-growing trend on the use of aluminium-alloys in shipbuilding and other marine services [1] due to features such as low weight, good dissolution resistance, and good mechanical properties [1,2]. The 5083 and 6082 Al-alloy series, which are the most used Al-alloys in maritime industry [1,2], exhibit yield strength requirements equivalent to that of steels [1] in addition better corrosion and mechanical properties relative to other grades [2].
The process of gradual cooling of a sample from room temperature to extremely low temperature and to room temperature again is referred as to cryogenic treatment [3][4][5][6]. The sample is allowed to stay at the extreme temperature for a long duration and then gradually, it is brought back to normal temperature [3][4][5][6]. During this process, the sample shrinks causing the sample's atoms to be rigidly held together [7]. The process could be termed shallow and deep depending on the temperature at which it is carried out [3]. For shallow cryogenic treatment, the sample is frozen to about −80°C [2,8,9] whereas, in deep cryogenic treatment, the sample is cooled to temperature as low as −196°C [5,6,10,11]. It had been reported that, cryogenic treatment improves material properties and durability [2,3,7]. Pan et al [11] conducted experiments to ascertain the contributions of deep cryogenic treatment to the microstructure and deterioration behaviour of a microarc oxidized Mg-2.0Zn-0.5Ca alloy. The process was found to have a refining influence on the microstructure and the grain size of the alloy; induced phase condensation and as well created crystal defect states in the examined alloy. Haramritpal et al [10] investigated the extent to which subjecting a Friction Stir Processed (FSPed) magnesium alloy to cryogenic process will have on the mechanical property and on the corrosion characteristics in sodium chloride solution. The corrosion resistance and mechanical property of the cryogenically treated magnesium alloy samples were found to improve remarkably relative to that of the control. The improvement was associated with the scattering of particles with deep cryogenic treatment. Uygur et al [2] reported improved dissolution resistance characteristic for AISI D3 steel in chloride-enriched solution occasioned by cryogenic treatment. Baldissera and Delprete [6] reported an enhanced fatigue property and no effect on hardness property of solubilized AISI 302 after deep cryogenic practice. Similarly, cryogenic practices have been found to be beneficial in protecting magnesium alloys against stress corrosion cracking in simulated body fluid [12].
The debate on which cryogenic treatment method is better, shallow or deep and the influence of cryogenic time on the process is ongoing. Senthilkumar et al [8], in their comparative studies on the effect of the two types of cryogenic practices on the residual stress state of 4140 steel claimed that shallow process encouraged tensile state of residual stress while the deep process showed a compressive residual stress. Arunram et al [9] reported improvement in the hardness property of steel grade M2 drilling tool by 40.96% through deep cryogenic treatment for 30 h and an increase of 31% in hardness by shallow cryogenic treatment for 30 h. Sonar et al [3], in their review advocated for deep cryogenic treatment of materials and a soaking period of 24 h in expense of shallow treatment process. However, in the investigation of Gao et al [5] on the contributions of cryogenic duration (2 h, 12 h and 24 h) to microstructure, mechanical, and dissolution resistance properties of WC-Fe-Ni cemented carbides noted that, during deep cryogenic treatment at -196°C, the corrosion resistance property of the cemented carbides diminished. The study revealed that, prolonging treatment duration ensured phase conversion, which in turn improved the studied properties.
As part of our contribution to this area of research, this work is designed to examine the effect of shallow cryogenic treatment (-80°C) and cryogenic time (10 h, 24 h, 36 h, 48 h, and 72 h) on the hardness, surface roughness, and corrosion resistance properties of AA5083-H111 alloy in a chloride-enriched system. The hardness property of the alloy before and after cryogenic treatments is analysed by Brinell hardness measurement. The corrosion resistance property is realized by studying the electrochemical behaviour of the alloy and the morphological changes on the alloy surface before and after exposure to studied medium. The electrochemical impedance spectroscopy (EIS) technique is used for electrochemical studies while the SEM and AFM techniques are used for morphological studies.
2. Material, chemical, and experimental procedures 2.1. Material and chemical preparation The material considered in this study is AA5083-H111 alloy metal. It was procured from ALRO SA, Romania and in table 1 is listed the weight percentage content. The alloy with a wall thickness of 6 mm was mechanically cut into rectangular shape (20 mm×40 mm). The samples for corrosion studies were encapsulated to expose only 0.785 cm 2 as the working area. The corrosion medium was 3.5 wt.% NaCl solution prepared by dissolving appropriate amount of NaCl salt (Sigma-Aldrich) in distilled water.

Cryogenic treatment of samples
Prior to the cryogenic treatment process, the AA5083-H111 alloy samples were mechanically abraded using various grits (400, 600, 800, and 2000) of abrasion papers. Thereafter, the samples were washed in distilled water and acetone so as to remove the dusts generated from the abrasion process and finally dried in warm air (about 40°C). The prepared samples were subjected to shallow cryogenic treatment by placing the samples in a Core DF 490 device and slowly cooling from room temperature to −80°C. The samples were soaked at −80°C for different durations (10 h, 24 h, 36 h, 48 h, and 72 h) in order to ascertain the influence of cryogenic time on studied properties. At the end of the specified time, the samples were kept at room temperature for 2 h before measurements.

Brinell hardness measurement
The hardness property of AA5083-H111 alloy samples un-cryogenically and cryogenically treated were measured using a DIGIROCK-RB hardness tester. A load of 100 kg was applied on each sample with the help of an 1/18 inch diameter ball system and with a dwell period of 5 sec. Each experiment was repeated for seven times and the average value is presented in this work.

. EIS experiments
The dissolution resistance property of AA5083-H111 samples un-cryogenically and cryogenically treated in the chloride-enriched solution were studied using the EIS technique. Experiments were performed on a Gamry Reference 600 potentiostat/galvanostat/ZRA instrument. The experimental setup consisted of the AA5083-H111 sample as the working electrode, a platinum rod as the counter electrode, and a Ag/AgCl reference electrode. Before each electrochemical measurement, the working electrode was trapped in test solution for 21600 s. Impedance measurements were done at the frequency range of 0.1 Hz to 100000 Hz and amplitude of 10 mV peak to peak. All electrochemical corrosion studies were carried out at room temperature and each experiment was repeated for seven times under same conditions to ensure reproducibility. The impedance analysis was made with the help of a ZsimpWin 3.21 software and the selected equivalent circuit is shown in figure1.

Surface analysis experiments
After the EIS experiments were performed on AA5083-H111 alloy samples, the samples surfaces were examined using SEM, energy dispersive x-ray spectroscopy (EDX), and AFM. The SEM and EDX analysis were performed using a J. Quanta FEG 250 (FEI, Holland) model device. The instrument was accelerated at 10 kV. The AFM analysis was achieved with PARKSYSTEMS, XE-100E model (UK) instrument.

Analysis of surface corrosion products
The AA5083-H111 alloy contains magnesium as the major alloying element and several other elements (table 1). The presence of these alloying elements give rise to second phase particles (Al 6 Mn, Al 6 MnFe, Mg 2 Si, Al 3 Mg 2 , Al-Si, Al-Mn, Al-Fe, Al-Cr, etc.) formation within the metal composite parts and along the interface between grains [13] which the variance in the electrochemical potential of the matrix and the second phases makes Al alloys to be prone to corrosion of different forms including the localized pitting corrosion [14][15][16]. For the AA5083 alloy grade, two parallel corrosion processes are proposed [13,17]. Firstly, corrosion is based on the cathodic stimulation of the Al part by the rapidly anodic dislodging of Fe-rich intermetallic particles like Al 6 MnFe, Al-Fe, etc. [13,17]. Secondly, it is based on the chemical and electrochemical activities of Mg 2 Si, that often result in the selective leaching of Mg and the precipitation of SiO 2 ·nH 2 O and/or magnesium hydroxide [17]. Figure 2 depicts the SEM micrographs of the as-received AA5083-H111 alloy at (a) 2000 magnification and (c) 10,000 magnification after exposing to the studied chloride-rich medium at 25°C for 24 h. The associated EDX spectra are shown in figures 2(b) and (d), respectively. The elemental mapping of the corroded surface is shown in figure 2(e). Obviously, the alloy surface corroded severely in the chloride-rich solution. In figure 2(a), a severely damaged surface with different sizes of pits is seen. Similar localized pitting corrosion was reported by Beura et al [13] for dynamically deformed Al-Mg (AA5083) alloy in 0.6 M NaCl solution. It is believed that defects on aluminium matrix are responsible for the growth of oxide layers that serve as the initiation sites for chloride ion absorption in a chloride-rich medium [14][15][16]. The Point defect model [18,19] stipulates that the initiation of pitting corrosion in Al alloys is the attachment of Cl − ions on anion vacancies of oxide layer. The inspection of the EDX spectrum in figure 2(b) reveals that the alloy surface is rich in chlorides and oxides. The adsorption of the chloride ions on the oxide layer may have shrinks the corrosion resistance property of the alloy [16]. It is also true from figures 2(c)-(e) that the Al-Fe and Al-Mn second phases contributed significantly to the corrosion of the alloy in the studied medium. As could be seen in figures 2(d) & (e), Fe and Mn are the major Figure 1. Equivalent circuit used for the analysis of electrochemical impedance data. R s is the solution resistance, R f is the film resistance, R ct is the charge transfer resistance, and Q is the constant phase element (CPE).
components of the corrosion products conforming with the report of others [13,17] that AA5083 alloy grades corrode mostly along these second phase particles. Generally, the idea of metals cryogenic treatment is based on two main reasons: (a) to eliminate retained austenite [2,12] and (b), to initiate nucleation sites for the condensation of fine particles [2,12]. It is also believed that cryogenic treatment slows down atomic level movements within a metal system, which in turn diminishes the system energy causing the metal to be chemically less reactive to its environment [3]. It is obvious that the pitting corrosion observed in figure 2(a) is reduced in figure 3(a). The surface in figure 3(a) is rather covered with products, which the EDX spectra in figures 3(b) & (d) reveal are different from the deposits found on the surface of the un-treated alloy ( figure 2(a)). The product is enriched in Na (figures 3(b) & (d)). It could be the precipitated particles that decreased the surface area for the ingression of chloride ions into the metal. Uygur et al [2] had reported the improvement of the deterioration resistance property of AISI D3 steel in sodium chloride solution after shallow cryogenic treatment occasioned by the precipitation of secondary fine particles. Ramesh et al [20] documented that deep cryogenic practice encouraged the settling of secondary fine particles on structural steel samples and this enhanced the corrosion resistance of the steel. It however, appears from figures 3(a) & (e) that the precipitated products did not cover the entire surface. In fact, by closing inspecting      figure 5(a). This observation which is in conformity with previous reports [20,24,25] indicates least vulnerability of the treated surfaces to corrosion attack [20]. According to Ramesh et al [20], the rougher topography of the untreated surface translates to higher corrosion susceptibility and is due to the absence of precipitates.

Analysis of surface roughness
The R a value is very valuable when it comes to checking the quality of a material surface [25]. Generally, the smaller the R a value, the higher the surface quality. A surface with considerable small value of R a is deemed to have lesser defects and free from adhesion of foreign particles [20,25]. It is also believed that homogeneity of a surface can limit surface corrosion attack [20,25]. In this work, R a value for the untreated AA5083-H111 alloy after corrosion in NaCl solution is 534.000 nm. The value is reduced to 300.472 nm, 285.634 nm, 105.362 nm, 177.908 nm, and 140.140 nm on surfaces cryogenically treated for 10 h, 24 h, 36 h, 48 h, and 72 h, respectively. This corresponds to reduction by 43.73%, 80.21%, 52.74%, 66.68%, and 73.76%, respectively. Similar significant reduction in R a value after cryogenic treatment was observed by Agrawal et al [25] and was interpreted to mean improvement of surface properties. Based on the R a value, 24 h is the best cryogenic treatment time. It is in conformity with the report of Özbek et al [26]. The authors had examined the role of cryogenic practice at various retention times (12 h, 24 h, 36 h, 48 h and 60 h) on cemented carbide inserts on tool wear and found best wear resistance for 24 h cryogenically treated samples. Again, the observation was attributed to better improvement of surface properties of the cemented carbide inserts at 24 h of treatment.

Analysis of corrosion resistance
To acquire insight into the effect of cryogenic treatment on the dissolution resistance property of AA5083-H111 alloy in the studied environment, the Open-Circuit Potential (E ocp ) of the as-received, 24 h, and 72 h cryogenically treated AA5083-H111 alloy samples was monitored at 25°C for 21600 s. The results obtained are presented in figure 6(a). Both the as-received and the treated alloy samples behave alike at the beginning of the experiments. The E ocp of the as-received, 24 h, and 72 h cryogenically treated samples begins at −0.87 V versus Ag/AgCl, −0.73 V versus Ag/AgCl, and −0.83 mV versus Ag/AgCl, respectively and progresses anodically for about 2500 s before flattering ( figure 6(a)). Such progression, which had been reported by others [27][28][29] is due to the premier deterioration of the pre-exposed, air-formed oxide layer, and the attack on the alloy samples surfaces [27][28][29]. Compared to the E ocp of the as-received, the E ocp of the cryogenically treated samples is displayed toward nobler potentials indicating higher resistance of the treated samples to corrosion processes [27]. It means that the cryogenic treatment improves the corrosion resistance property of the alloy. By comparing the E ocp of the 24 h and 72 h treated samples, the 24 h treated sample is found to exhibit nobler potential than the 72 h treated sample. For instance, the E ocp at 21600 s for the 24 h treated sample is −0.66 V versus Ag/AgCl while that of the 72 h treated sample is −0.68 V versus Ag/AgCl. This points to superior resistance to corrosion by the 24 h cryogenic treated sample than the 72 h treated sample. These results are in consonance with the AFM results ( figure 5). At the end of the open circuit potential experiments, the impedance characteristics of the alloy samples were collected. Figure 6(b) shows the impedance spectra of AA5083-H111 alloy without and with cryogenic treatment in the studied medium at 25°C. Depressed semicircles are observed in all cases and is a characteristic of a metallic electrode whereby dissolution process is driven by a charge transfer [27,30]. The capacitive loops in both the asreceived and cryogenically treated samples are same and indicates same mechanism of corrosion [27]. However, the treated samples exhibit bigger capacitive loops relative to the as-received indicating higher corrosion resistance [27,31,32]. Again, the capacitive loop of the 24 h treated alloy sample is wider than that of the 72 h treated sample agreeing with other results (figures 5; 6(a)) that the best cryogenic treatment time for the studied substrate is 24 h. The values of the solution resistance (R s ), the components of constant phase element (Y 0 and n), the charge transfer resistance (R ct ), and the resistance of the surface film (R f ) obtained from the analysis of the impedance data using the equivalent circuit given in figure 1 are summarized in table 2. The total charge transfer resistance, R p (i.e R ct +R f ) of the as-received, 24 h, and 72 h cryogenically treated AA5083-H111 alloy samples is 18139 Ω cm 2 , 26230 Ω cm 2 , and 19191 Ω cm 2 , respectively. This translates to corrosion resistance enhancement of 30.85% and 5.48%, respectively by cryogenic treatment for 24 h and 72 h, respectively. Although cryogenic treatment cannot be recommended as a first choice corrosion control measure in expense of other techniques like surface coatings, it should be mentioned that the beauty of cryogenic technique lies on the fact that corrosion control can be achieved without the metal losing its mechanical and other desired properties [3,20,33]. Ramesh et al [20] suggested the use of cryogenic treatment on metals meant for special applications like aerospace, freezethaw region, etc. The value of Y 0 provides information on the characteristics of a surface film [34]. A small value of Y 0 points to good features of a surface film [34]. The Y 0 values in table 2 reveals better surface film characteristics for the 24 h cryogenically treated surface than the untreated and 72 h treated surfaces. Also worthy of pointing out from table 2 is the fact that both n1 and n2 values for all the surfaces are near unity and this is reflective of capacitive surfaces [35].

Analysis of hardness
Numerous works have highlighted the positive effect of cryogenic treatment on hardness property of metals. Arunram et al [9] reported 16.59% and 25.21% improvement in the hardness of high-speed steel grade M2 drilling tool after shallow and deep cryogenic treatment for 24 h. Amini et al [36] observed 4.6% increase in hardness of 80CrMo125 tool steel after deep cryogenic treatment. Similarly, Harish et al [37] noted 14% and 13% improvement in the hardness of EN31 bearing steel after deep and shallow cryogenic treatments, respectively. In all these reports, the improvement in hardness property was linked to a reason earlier mentioned -the conversion of hold on austenite to another form (martensite) and the settling of fine secondary particles [9,36,37]. 3.5. Possible mechanism for the observed improved corrosion resistance and hardness Cryogenic treatment influences the microstructure of a material in three ways: (i) transformation of retained austenite into martensite, (ii) precipitation of finely dispersed particles, and (iii) relaxation of residual stresses [10,20]. In most cases, the second effect, that is, precipitation of finely dispersed particles is responsible for the improvement in corrosion resistance property after cryogenic treatment [22,23]. The fine precipitates are formed in the outer wall microstructures and can interact with dislocations, resist dislocation motion [23], and form Guinier-Preston (GP) zone [38]. The GP zone can ensure enhancement of materials properties. Franco Steier et al [39] attributed the reduction of wear rates of specimens to the generation of GP zone in the cryogenically treated specimen. Syed et al [23] observed that the formation of this zone led to the improvement of hardness property of Al 6101 closed-cell foam after cryogenic treatment. The observed increase in the hardness property of the studied AA5083-H111 alloy could as well be attributed to the action of the precipitated particles.
For the improvement of corrosion resistance property, precipitated particles depending on the volume, type, size, distribution pattern, and shape, can restrict the penetration of corrosive species and in turn decrease the rate of corrosion [22]. Recently, it was reported [23] that Al x Fe x Si x and Mg 2 Si are the main components of the precipitates on a cryogenically treated 6101 aluminium alloy exposed to 3.5 wt.% NaCl solution. As earlier mentioned, the studied AA5083-H111 alloy contains alloying elements (table 1) that give rise to second phase particles (Al 6 Mn, Al 6 MnFe, Mg 2 Si, Al 3 Mg 2 , Al-Si, Al-Mn, Al-Fe, Al-Cr, etc). Our EDX results (figures 4(e)-(h)) reveal a significant increase in the percentage composition of Al and Mg on the precipitated particles on the cryogenically treated samples surfaces compared to the untreated sample surface (figures 2(b) & (d)). The precipitation of this particles is the reason for the observed corrosion resistance enhancement. Additionally, nano-sized grains that are uniformly distributed can induced the formation of a passive oxide film and this will render the metal surface more electrochemically stable [10,20,22,23]. The less surface roughness of the cryogenically treated surfaces also contributes to the improved corrosion resistance of the alloy. The reduction in surface roughness strengthens the resistance of the surface to corrosive attacks.

Conclusions
Cryogenic process involves the gradual freezing of specimen to extreme low temperature, retention at that temperature for as long as 24 h, and reversal to room temperature [3][4][5][6]. It has been demonstrated that cryogenic treatment improves certain properties of material [3][4][5][6]. In this work, AA5083-H111 alloy samples were subjected to shallow cryogenic treatment at -80°C for 10, 24, 36, 48, and 72 h. The effect of the process on the hardness and surface roughness properties of the alloy as well as on the corrosion behaviour of the alloy in 3.5 wt. % NaCl solution was studied. The following deductions are made.
1) The shallow cryogenic treatment improves the hardness, surface roughness, and corrosion resistance properties of AA5083-H111 alloy.
2) The best shallow cryogenic treatment time for the alloy is 24 h.
3) The cryogenic treatment of AA5083-H111 alloy for 24 h increases the hardness property by 4.81%, reduces surface roughness by 80.21%, and improves corrosion resistance by 30.85%.
4) The SEM results confirm the settling of fine particles on the surfaces of the alloys samples in agreement with the results of Syed et al [23] that Al x Fe x Si x and Mg 2 Si precipitates settled on the surface of a cryogenically treated 6101 aluminium alloy after immersion in 3.5 wt.% NaCl solution and improved corrosion resistance property.

Data availability statement
The data that support the findings of this study are available upon reasonable request from the authors.

Funding
We acknowledge the financial assistance by the Duzce University Research Fund under the Project No: 2020.06.05.1111. The authors express their sincere gratitude to Prof. Dr İlyas Uygur for aiding in the supply of AA5083-H111 alloys.