VLIV STRUKTURY NA KOROZNÍ VLASTNOSTI HLINÍKOVÝCH SLITIN INFLUENCE OF STRUCTURE ON THE CORROSION PROPERTIES OF ALUMINIUM ALLOYS

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1 VLIV STRUKTURY NA KOROZNÍ VLASTNOSTI HLINÍKOVÝCH SLITIN INFLUENCE OF STRUCTURE ON THE CORROSION PROPERTIES OF ALUMINIUM ALLOYS Abstract Jiří FALTUS VÚK Panenské Břežany a.s., Panenské Břežany 50, Odolena Voda The paper deals with studying the impact of structural properties and metallurgical state of the corrosive properties of pressed flat bars of the commercial aluminium alloy AA6082, AA6262, AA6012, AA2014, AA 2007, and new lead-free aluminium alloys AA6023 and AA2015. Samples with surface recrystallised layer (SRL) or without it were exposed to the corrosive atmosphere NSS according to the ISO The surface profiles were either the original state or anodizing selected mode. Form of corrosion attack and depth of corrosion attack are performed depending on the type of alloy, the metallurgical temper and of the structure of surface areas. 1 INTRODUCTION According to the position in a number of electromotive voltage Aluminium is thermodynamically reactive metal. Among structural metals are beryllium and magnesium, only more reactive. Excellent corrosion resistance of aluminium is associated with Oxide film (oxide layer) firmly tied to the surface. In most environments, the violation of the oxide layer again very quickly and immediately restored after damage. The freshly scoured the surface of aluminium after exposure to air is oxidického barrier film only about 1 nm (10 Å) thick, but it is very effective in protecting aluminium from corrosion. Oxide layer (oxide film), which is in normal atmosphere in thicknesses much larger than 1 nm (10 Å), is composed of two layers [2]. The inner oxide layer (the so-called barrier layer), closer to the metal is a compact amorphous layer, whose thickness is determined solely by a temperature environment. For a given temperature limit is the same thickness of the barrier in an environment of oxygen, dry air, even in an environment of humid air. The total surface oxide layer is thicker and more permeable the so-called outer layer hydrated oxide. Many interpretations of the processes of corrosion in aluminium and its alloys is based on the chemical properties of these two layers. Establishment oxide layer can be thought of as dynamic equilibrium between two opposing tendencies: i) the tendency of a compact barrier layer and ii) a tendency to its breaking. In the absence of destructive forces created in dry air only a single layer. Until there is a limit thickness of this layer very quickly. If the destructive power is too strong a rapidly hydrated, leading to the emergence of a relatively thin barrier layer. Between these extremes, the relative thickness of the oxide film ranges from 20 to 200 nm (200 do 2000 Å). Terms of thermodynamic stability oxide layer expresses Pourbaixův diagram (potential versus ph) (fig. 1). According to diagram the aluminium is passive (protected Oxide film) in the ph range of 4 to 8.5. Defining this range varies with temperature, with the specific conditions of oxide layer and with the presence of substances that may affect the emergence of complex or insoluble salt. The fig. 2 shows weight loss of samples alloy AA3004-H14, exposed of water-salt solutions with different ph value. The curve shows that the alloy is passive in the ph range 4 to 7 1

2 NSS AASS Fig. 1 Pourbaixuv diagram for aluminium Ref. [4] Fig. 2 Loss of weight alloy AA3004-H14 after exposure to 1 week in distilled water and solutions with different ph values Ref. [4] Aluminium corrodes in water solutions outside the defined passive area. t does so because the oxides are soluble in acids and principles, while generating ions AL3 + and AlO2-. In cases of failure to dissolve the surface oxide layer, or if oxygen is present, the corrosion of aluminium does not occur either inside or outside the field of passive [3] Types of corrosion. For aluminium and its alloys are encountered in a number of different types of corrosion attack such as galvanic corrosion, deposition corrosion. intergranular corrosion, pitting corrosion, stress-corrosion cracking (SCC corrosion), exfoliation corrosion, erosion-corrosion attack, atmospheric corrosion, corrosion filiform corrosion and corrosion fatigue. Detailed description of the different types of corrosion is in [4, 6]. Due to the needs of this work, we focus on a detailed description of the two types of corrosion on the pitting and inter-granular corrosion. Both types of corrosion are observed in the corrosion tests in atmospheres of NSS and AASS. Pitting corrosion is caused by local corrosion attacks, resulting in the creation of cavities. The pitting corrosion is required an electrolyte, either in the form of a liquid, mist or thin film of moisture on the surface. For the emergence of pitting corrosion is also required the presence of oxygen. Shape of pits can vary from broad and shallow pitting to deep and narrow cavity (obr.3). In the early stages of the growing point of failure called autocatalysis [6, 7]. The emergence of pit defects is associated with a violation of the surface oxide layer in areas adjacent to the cathodic types of particles. If the copper ion is present, it will grow cathodic space and managing the potential begins to increase. Development point cavity (anode) will take place when the acidity reaches the corrosive environment of ph values 3 to 4 and the cathodic protection of the particle is slightly alkaline. These local changes in the environment contribute to the growth potential of managing the emergence of cavities. At some stage, reach a stationary state of the tendency of pitting remains constant. The growth of cavities during the pitting corrosion is strongly affected the composition of the electrolyte. Influence of anions and cations in the complex is not yet fully understood [8]. 2

3 The method of evaluation pitting corrosion. Pitting corrosion is evaluated either visually or metallographically to cut the sample. On the surface contested the corrosion is determined by the shape, density, size and depth of pits using standardized procedures (Fig. 3B). A) Fig. 3A Variations in the cross-sectional shape of pits, a) Narrow and deep, b) Elliptical, c)wide and shallow, d) Subsurface, e) Undercutting, f) Shapes determined by micristructural orientation Fig. 3B Standard rating chart for pits by ASTM Ref [5] Ref [5] Intergranular corrosion is again the local corrosion, which attacks the grain boundaries and the areas around them. In the case of fibrous structure the subgrains are attacked. In this case we speak about interfragmentary corrosion [3]. The mechanism of the corrosion is electrochemical. It is the result of differences in electrical potential between the grain boundaries and matrix. Areas with different potentials are arising between microstructure and secondary phases of the surrounding matrix, which is composed of solid solution of alloy elements. In some alloys such as Al-Mg (5xxx group) and Al-Zn-Mg-Cu (7xxx group) are secondary phases Mg 5 Al 8, MgZn 2 and complex phases Al x -Zn y -Mg z, which are more anodic than the surrounding matrix. Contrast, in Al-Cu alloys (2xxx group) are phases CuAl2 and Cu x Mg y Al y more cathodic than the surrounding matrix. Contrast, in Al-Cu alloys (2xxx group) are phase CuAl 2 and Cu x Mg y Al y more cathodic than the surrounding matrix. In both cases, the local corrosion attack is linked to the grain boundaries and the closest surroundings. Tendency to intergranular corrosion depends on the technology of production and treatment of alloys, since the method of processing significantly affects the distribution and nature of secondary phases in the structure of aluminium alloys. Better resistance to intergranular corrosion be achieved either by using such technologies of heat treatment, which causes the maximum uniformity in the distribution of phases in the structure of alloys or by limiting the content of alloying elements, which lead to reduce the number of secondary, so-called constitutional phases in the structure of alloys. Method evaluation tendency to itergranular corrosion depends on the type of alloys. Metallographic evaluation after exposure to the corrosive solution of NaCl-H 2 O 2 (see standard MIL-H-6088F) is used particularly for 2xxx group alloys (Al-Cu-Mg), 6xxx (Al- Mg-Si) and 7xxx (Al-Zn-Mg-Cu). For 5xxx group alloys (Al-Mg) was developed the method of measuring the weight loss (NAWLT) according to ASTM G To estimate the propensity to intergranular corrosion for specific 2xxx and 7xxx alloys are sometimes used 3

4 electrochemical techniques (corrosion potential and galvanic methods).confirmation of the conclusions using of the methods of light microscopy is still necessary. 2 METHODOLOGY AND PARAMETERS OF CORROSION TESTS Corrosion tests were carried out in two artificial atmospheres: a) in the atmosphere NSS b) in the atmosphere ÀÄSS according to ČSN ISO 9227 with exposure time 240 hours. Atmosphere formed by spraying solutions prepared according to ISO 9227, which was the basis of sodium chloride solution prepared from distilled water. When tested in an atmosphere of NSS was in the test chamber at 25 C spraying sodium chloride solution with ph ranging from 6.5 to 7.2 ph. When tested in the atmosphere ÀÄSS with sodium chloride added an appropriate amount of glacial acetic to achieve ph between 3.1 to 3.3 ph (Fig.1). It tested a total of nine commercial alloys of aluminium and alloys AA6082, AA6262, AA6023, AA6012 and AA6026 in the temper T5, alloys AA2011, AA211B in the temper T6 and alloys AA2007, AA2015 in the temper T4. Corrosion was tested on the transverse sections of rods with diameters from 75 to 85 mm. The surface of samples was either in the original state (fine milling) or was eloxal coated in optimal conditions. For each variant of corrosion tests were exposed to 5 samples. After exposure and treatment according to ČSN ISO 9227 have exposed samples evaluated using standardized procedures according to EN ISO Evaluation of samples was carried out in the workplace VZLU, Inc., where the samples were exposed in corrosion chambers. The results are summarized in the test [1]. 3 RESULTS AND DISCUSSION 3.1 Mode of corrosion attack Result from corrosion attack can be divided into two basic groups. Pitting corrosion was observed with different shape and depth of pits and intergrradual corrosion was observed in recrystallised structures of alloys AA2011 and AA2111B. Interfragmentary corrosion was in the fibrous structures of other alloys. Both modes of attack have been developed in various degrees and depths depending on the type of alloy, on the surface treatment and used corrosive environments. Pitting corrosion was detected in all tested alloys. a) b) Fig. 4 Pitting corrosion on the original surface after corrosion in NSS a) AA6262-T5 and b) AA6023- T5 4

5 i) For 6xxx alloys in the environment NSS was usually observed a wide and shallow dimples (Fig. 4). If the surface of samples in the original state (fine milling), sometimes occurring deeper pits elliptical shape or narrow and deep pits. When using the atmosphere ÀÄSS the incidence of deep and narrow pitting were increased and also were observed underground pits and dimples with the orientation by microstructure. Anodic oxidation significantly reduced density, size and depth of pitting corrosion. ii) For 2xxx type alloys the pitting corrosion was observed both on the surface of samples with the original surface and on surface of samples with anodised surface after exposure in corrosive environments NSS and ÀÄSS. Emergence pitting was not bound on the grain boundaries. On the surface samples of AA2007 and AA2015 alloys after exposure in the environment of NSS have been observed broad and shallow pits. Their origin is linked to areas where the secondary complex phase Al-Si-Fe-Mn, S-phase CuMgAl 2, CuAl 2 phase and soft phase containing low melting metal ascend to the surface (Fig. 5a). After exposure in the environment ÀÄSS was observed an increased incidence of deep narrow pits oriented with structure (obr.5b). With regard to corrosion attack appeared machinable lead alloy AA2007 more corrosion resistant than the new lead-free alloy AA2015. It can be assumed that this was due to higher density coarse undissolved phases CuMgAl 2 and CuAl 2 in the structure of alloy AA2015 alloy AA2007 compared. Intergranular corrosion. Exclusively to intergranular corrosive attack (besides the pitting corrosion) was observed only in alloy AA2011-T4 and samples with an unprotected surface in both test environments NSS and ÀÄSS and samples with anodic oxidation in an ÀÄSS (Fig. 6a). For lead-free alloys AA2111B not clean intergranular corrosion. In this alloy was observed the local volume corrosion which was linked to grain boundaries (Fig. 6b). a) b) Fig. 5 Pitting corrosion on the original surface after exposure a) AA2007-T4 in NSS and b) AA2015- T4 in AASS *** 5

6 a b) Fig. 6 Intergranular corrosion after exposure in ÀÄSS a) AA2011-T6 with anodised surface b) AA2111B-T6 with the original surface Remaining test group 2xxx alloys and 6xxx have a fibrous structure. These alloys were observed interfragmentary corrosion (Fig. 7). a) b) Fig. 7 Corrosion of the original surface fibrous structures after exposure ÀÄSS a) AA2015-T4, b) AA6012-T5 Effect of secondary (so-called constitutional) phases in the development of pitting and intergranular (interfragmentary) corrosion was for each type of alloys rather different. Establishment of pits on the surface of samples from 6xxx series alloys was bound mainly to the creation of complex intermetallic phases containing Al-Fe-Si-Cu or Al-Fe-Si-Mn. Growth of the pits or development intergranular corrosion took place along these stages, elongated in the direction of pressing. Phases containing low-melting metals Pb, Bi and Sn, which ran on the surface of the sample contributed to the emergence pits. It was on the ground that they phases violated or weakened surface oxide layer or anodized layer. Along these phases was not associated with deep pits. The emergence of pitting on the surface of samples alloys AA2011 and AA2111B and development intergranular the attack works regardless of the distribution phase containing low-melting metals (Pb, Bi and Sn) (Fig. 8a). For the test alloys the development of corrosion pitting, intergranular or interfragmentary corrosion originated primarily on gross cathodic phases containing Cu (CuAl 2 or CuMgAl 2 ) or Fe (complex phase of Al-Fe-Si-Cu-Mn) corrosion surrounding matrix (Fig. 8b). 6

7 Corrosion of the surrounding matrix Intermetallic phase CuAl 2 a ) b) Fig. 8 a) Development intergranular corrosion regardless of the dark phases of Pb and Bi, AA2011-T6 with the original surface, ÀÄSS b) Development of sub-surface pitting corrosion in the vicinity of phases CuAl 2 and CuMgAl 2, AA2011-T6 with the original surface, AASS 3.2 The depth of corrosion attack The depth of corrosion attack was evaluated using the methods of light microscopy. The results can be briefly summarized in several points: i) For all the test alloys ÀÄSS corrosive environment is more aggressive than the corrosive environment NSS (Table 1). ii) iii) iv) For samples with the original surface of the depth of corrosion attack in the NSS environment depended greatly on the type of alloy. The maximum depth of corrosion attack was the type of samples alloy Al-Cu, the smallest of samples from the group 6xxx alloys. During the test ÀÄSS corrosion the attack samples with the original surface was so aggressive that they had made no difference between the test alloys (Fig. 9a). The positive effect of eloxal coating on the depth of corrosion attack in the corrosion environment AASS has been particularly for the group 6xxx of alloys (AA6082, AA6262, AA6012 and AA6023). It turned out that the eloxal coating of alloys group 6xxx is not only the thickest and toughest, but best to protect the internal layers of material from corrosion in artificial atmospheres. The alloy AA6026 depth of corrosion attack was still high. It is obvious that in this alloy exhibited the influence of extremely high density of secondary, complex phases, the accumulation of high levels of Fe, Mn and Cu in particular. In the enclosure to the surface of these phases break the anodized layer. For these reasons, corrosion attacks were primarily in these places (Fig. 9b). Clearly showed that the all alloys with anodised surface after exposure to the atmosphere NSS had the least corrosion attack. In this group of alloys Al-Cu-type had the greatest depth of corrosion attack (max. 45 m). It is interesting that in terms of depth of corrosion attack, alloys Al-Cu-Mg with the eloxal coating is almost cope with the type of alloys Al-Mg-Si. 7

8 Table 1 Maximum depth of corrosion attack in environments NSS (A, C) and ÀÄSS (B, D) for nine types of alloys without any surface treatment (A, B) and after anodic oxidation (C and D) Metoda dle ČSN ISO 9227 NSS 1) AASS 2) NSS 1) AASS 2) Sample number Surface With the original surface eloxal coating Alloy and temper A B C D 1 AA6082-T AA6262-T AA6023-T AA6012-T AA2011-T AA2111B-T AA2007-T AA2015-T AA6026-T ) Test in a fog of neutral sodium chloride solution (NSS) 2) test in a fog of acidified sodium chloride solution (ÀASS) 600 Original surface 500 Anodised surface 450 Max.depth of attack [mm] Max.depth of attack [μm] Designation by AA B AASS NSS Designation by AA 2111B AASS NSS a) b) Fig. 9 The depth of corrosion attack in artificial atmosphere NSS and ÀÄSS for the test alloys 4 CONCLUSIONS a) For samples of alloys AA6082-T5, T5-AA6262, AA6023-T5, T5-AA6012, AA6026-T5, T5-6B3, AA2007-T4, AA2015-T4, AA2011-T6 and T6-AA2111B corrosion tests were conducted in artificial atmospheres NSS and AASS by ISO Samples were tested with the original surface and with eloxal coating surface. b) Testing of aluminium alloys can be divided into three groups according to depth and appearance of corrosion attack. i) Group 6xxx of alloys Al-Mg-Si with the best corrosion resistance ii) a group of duralumin type alloys Al-Cu-Mg with moderate corrosion resistance and iii) a group of binary alloys Al- Cu type with the lowest corrosion resistance. Alloys of the same group showed a similar appearance of corrosion attack. Depth of corrosion attack of alloys in the same group is not very different 8

9 c) Differences in corrosion attack of the alloy from one group of the test alloys have been caused by different contents of alloying elements and impurities in these alloys. Reduction of corrosion resistance of certain alloys 6xxx group was due to increased content of alloy elements and impurities, in particular the content of Cu, Mn and Fe. Corrosion resistant of the alloys decreased with increasing content of these elements. d) For duralumin type alloys AA2007 and AA2015 showed a somewhat better corrosion resistance of alloy AA2007-T4. This was due to higher content of secondary phase containing Cu in the structure of the samples of alloy AA2015-T4. Corrosion attack spread along the phases. The negative effect of Sn content in compared to the corrosion resistance of the alloys with content Pb failed to demonstrate. e) Alloys AA2011 and AA2111B varied manner corrosion attack. The alloy AA2011-T6 was observed significant intergranular corrosion, while AA2111B-T6 alloy was intergranular corrosion associated with large volume corrosion. The reason for the differences was the differences in the content of copper in both alloys. In the structure of alloy AA2011-T6 with a high content of Cu, copper was rejected at the grain boundaries, which supports the development of intergranular corrosion. In the structure of alloys AA2111B was much more even distribution of copper, resulting in volume corrosion. f) Atmosphere ÀÄSS (sodium chloride solution supplemented with glacial acetic acid with ph ranging from 3.1 to 3.3) was significantly more aggressive than atmospheric NSS (sodium chloride solution with a ph ranging from 6.5 to 7.2). g) Effect of anodized surface layer (eloxal coating) to improve corrosion resistance was significant for all tested alloys, especially in group 6xxx alloys. Protective effect of eloxal costing in the 6xxx alloys decreased with an increase in the content of Cu, Mn and Fe. The reason for it is the increased density of secondary phases containing the elements listed, which led to the deterioration of integrity of anodized surface layer and therefore decrease of its protective properties. ACKNOWLEDGMENTS This work was done as a part of project of Ministry of Education, Youth and Sports of Czech Republic, Research Centre 1M0556. Author would like to thank Alcan Decin Extrusions for the experimental material provided on these tests. REFERENCES [1] VZLU, Inc., Praha 9-Letňany, Test report No. 3445/07, Wrought aluminium alloys with good machinability, Corrosion resistance, [2] HUNTER, M.S. - FOWLE, P.: J.Electrochem. Soc., Vol.103,1956, p.482 [3] HATCH, J.E. Aluminium, Properties and Physical Metallurgy, ASM, 1984, [4] HOLLINGSWART, E.H. - HUNSICKE, H.Y.:Corrosion of Al and Al slitin, ASM Handbook, Vol.13, Corrosion, p.106 [5] Standard Recommended Practice for Examination and Evaluation of Pitting Corrosion, G 46, Annual Book of ASTM Standards, American Sociaty for Testing and Materials [6] EDLEANU, C.-EVANS, U.R.: Transactions of the Faraday Society, Vol.47, 1951, p.1121 [7] AZIZ, P.M.: Corrosion, Vol.9, 1953, p.85 [8] SOTOVDEH, K.: et al, Corrosion, Vol.37, 1981, p.358 9