Influence of Malonic Acid on the Corrosion and SCC Behaviour of Anodic Coated 1050 Al-Alloys

Abstract:

Aim of the present work is the study of corrosion and stress corrosion cracking behaviour of 1050 Al-Alloy anodised in a 3M H2SO4 anodising bath with the presence in it of malonic acid, in various concentrations and anodising current densities. The investigation was carried out by SCC (Stress Corrosion Cracking) tests and electrochemical measurements. The influence of applied potential on SCC behaviour was also examined. The corrosion and SCC behaviour of anodised 1050 Al-Alloy was found to vary with malonic acid concentration, anodising conditions, applied potential and stress level. In SCC conditions all prepared coatings protected the bare alloy, with better protective properties in the case of 0.015M concentration of malonic acid prepared with a 6 A.dm-2 anodising current density. The coating prepared in these conditions had better mechanical properties as indicated from the increased protection at a high stress level and also the better behaviour in corrosion, without stress, conditions of coatings prepared in different conditions of malonic acid concentration and anodising current density. For the interpretation of the results, properties of the anodic coatings as thickness, packing density, coating ratio, roughness, were also studied. The anodic coating formed in a electrolytic bath of 0.015M concentration of malonic acid and a 6 A.dm-2 anodising current density was found to be less porous, more compact and rough, with better oxide structure. Prepared coatings were found to increase protective properties in an area of applied potentials slightly more anodic than the free corrosion potential values.

• Periodical: Key Engineering Materials (Volume 438)
• Pages: 155-162
• DOI: https://doi.org/10.4028/www.scientific.net/KEM.438.155
• Online since: May 2010
• Authors: Panayotis Spathis, Efthimios Papastergiadis, Georgios Stalidis, Georgios Papanastasiou
• Keywords: Aluminum (Al), Anodic Coating, Corrosion, Malonic Acid, Protection, Stress Corrosion Cracking (SCC)

References

[26] In all cases the Al-Alloy is not passivated, as passive regions do not appear for potential values more positive than Epit and reverse currents are always higher than forward currents. It is also observed that an addition of 0. 015M of malonic acid prepared with a 6 A. dm-2 anodising current density shifted Epit in the noble direction and decreased the anodic current indicating less susceptibility to localised corrosion in the free corrosion potential regions. Fig. 3 Anodic potentiodynamic polarization curves of anodised in 3M H2SO4 1050 Al-Alloy and with an addition of various concentrations of malonic acid (1): 3M H2SO4+ 0. 005M malonic acid, (2): 3M H2SO4 + 0. 015M malonic acid, (3): 3M H2SO4+ 0. 03M malonic acid, (4): 3M H2SO4 +.

DOI: 10.4028/www.scientific.net/kem.438.155

05M malonic acid, (5): 3M H2SO4 + 0. 1M malonic acid. Fig. 4. Anodic potentiodynamic polarization curves of anodised in 3M H2SO4+ 0. 015M malonic acid 1050 Al-Alloy with various anodising current densities : (1): 1. 5 A. dm-2, (2): 3. 5 A. dm-2, (3): 6 A. dm-2, (4): 7. 5 A. dm-2 , (5): 10 A. dm-2 The results of the study of the influence of the application of a controlled potential during SCC tests are shown in Fig. 5. From these results follows that in all applied potentials, from -1. 7 V to +0. 2 V, greater times to failure are observed in the case of malonic acid. In all cases of tested specimens, bare or coated, areas of increased TTF are observed in both anodic and cathodic directions in the regions near the free potential values, but TTF decreases for more anodic or cathodic direction. A greater value of TTF is observed in the case of malonic acid in an area of applied potential slightly more anodic than the corresponding free corrosion potential value. Fig. 5. Influence of applied potential to time to failure for bare (1), anodised in 3M H2SO4 (2), anodised in 3M H2SO4+ 0. 015M malonic acid (3), 1050 Al-Alloy. Free corrosion potentials are shown (0). The measurements of some physical properties of the anodic coatings shown in Table I indicate that the addition of malonic acid during anodising decreases thickness and increases packing density of the coatings resulting in the formation of a less porous oxide layer. The coating ratio decreases in the presence of malonic acid, while roughness increases. This latter result can be explained from the possible dissolution of the outer surface of the oxide during anodising, as it was found in earlier work [19]. Table I Some physical properties of anodic coating prepared in 3M H2SO4 in presence of various concentrations of malonic acid. Anodizing bath composition oxide thickness (µm) packing density (gr/cm3) coating ratio roughness (µm) 3M H2SO4 11. 864 2. 555 1. 42 0. 258 3M H2SO4+0. 005M malonic acid.

[11] 863 2. 628 1. 37 0. 260 3M H2SO4+0. 015M malonic acid.

[10] 232 2. 914 1. 39 0. 289 3M H2SO4+0. 03M malonic acid.

[11] 466 2. 671 1. 38 0. 268 3M H2SO4+0. 05M malonic acid.

[10] 653 2. 778 1. 34 0. 270 3M H2SO4+0. 1M malonic acid.

[11] 709 2. 721 1. 28 0. 280 CO CLUSIO S.

[1] The corrosion and SCC behaviour of anodised 1050 Al-Alloy was found to vary with malonic acid concentration, anodising conditions, applied potential and stress level. In SCC conditions all prepared coatings protected the bare alloy, with better protective properties in the case of.

015M concentration of malonic acid prepared with a 6 A. dm-2 anodising current density.

[2] The coating prepared in these conditions had better mechanical properties as indicated from the increased protection at a high stress level and also the better behaviour in corrosion, without stress, conditions of coatings prepared in different conditions of malonic acid concentration and anodising current density.

[3] The addition of a 0. 015M concentration of malonic acid with a 6 A. dm-2 anodising current density during anodising decreases thickness and increases packing density of the coatings, resulting in the formation of a less porous and more compact oxide layer.

[4] Prepared coatings were found to increase protective properties in an area of applied potentials slightly more anodic than the free corrosion potential values. REFERE CES.

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