ISSN 0010-9312 (print), 1938-159X (online) 13/000221/$5.00+$0.50/0 © 2013, NACE International
A test assembly consisting of a coated and scribed aluminum alloy 7075-T6 panel galvanically connected to uncoated through-hole noble fasteners has recently been utilized to quantify corrosion attack during corrosion testing in laboratory chambers. In this work, the corrosion morphology and extent of attack of Al alloy panels with five different surface pretreatments prior to painting and scribing were compared after exposure to ASTM B117 with galvanic connection to uncoated stainless steel fasteners. The nature of the attack for samples with different surface pretreatment samples was found to exhibit two different trends: they either penetrated deeply into the substrate at the scribes or spread out under coatings. The galvanic currents between the coated Al alloy panel and the bare 316 stainless steel fasteners were monitored during 21 days exposure in an ASTM B117 chamber. The current trends measured for different surface pretreatment panels were consistent with optical profilometry (OP) results. However, the galvanic current and OP techniques both underestimated the extent of corrosion attack because of local H2 evolution and undercut attack, respectively. The results from both techniques were modified to compensate for these deficiencies. Acceleration factors associated with the galvanic interaction for the different surface pretreatment panels were determined.
A test sample incorporating a painted Al alloy panel, uncoated through-hole fasteners, and scribes has recently been shown to provide accelerated response during atmospheric corrosion testing in the field and in laboratory chambers. In this paper, the galvanic current of an AA7075-T6 panel coupled with mixed SS316 and Ti-6Al-4 V fasteners was monitored using a zero-resistance ammeter during 3 weeks exposure in an ASTM B117 chamber or immersed in 5 wt% NaCl solution. SS316 fasteners provided more cathodic current than Ti in both environments and the current in ASTM B117 is higher than in 5 wt% NaCl solution due to greater oxygen availability. The integral of the anodic current with time and optical profilometery (OP) analysis were used to assess the corrosion attack quantitatively for two different coating systems. An acceleration factor was defined to represent the extent of accelerated corrosion for galvanically-connected fasteners. The acceleration factors were in the range of 20-50 for panels with SS316 fasteners and two different coating systems, both with and without a topcoat. The effects of SS316 fasteners were similar for the different coating systems even though the attack morphology was very different. Aluminum alloy (AA) 7075-T6 (UNS A97075) is a high-strength aluminum alloy that is widely used in structural aircraft applications due to the combination of good mechanical properties and light weight.1 However, AA7075-T6 is very susceptible to localized corrosion, including pitting, intergranular and crevice corrosion when exposed to an aggressive environment.2-5 As a result, aluminum alloys for aircraft applications are usually protected from the corrosion attack using multi-layered coating systems.6-8 Because of the generally good corrosion resistant properties by coating systems, a long period is required to observe their degradation or failure even when exposed to aggressive environments, which makes comparison of different coating systems difficult. Therefore, a new accelerated corrosion test sample was design to be efficient and rapid for assessing coatings.9,10 It incorporates a painted Al alloy panel, uncoated through-hole noble fasteners, and scribes in the coating under the fasteners. The introduction of noble fasteners in such sample design activates galvanic corrosion and provides cathodic current to drive severe corrosion attack at scribes after only 567 h exposure to ASTM B117. Galvanic current, morphology and corroded volume data were provided, 9,10 but quantitative descriptions of the acceleration extent for galvanically-connected fasteners were not discussed. A quantitative method for assessing degradation is critical to be able to use these new test specimens in research and development, materials specifications, and assessments for the life of coating systems.Many conventional methods for assessing coatings have been studied in recent decades, including ASTM B117 exposure, field exposure, electrochemical impedance spectroscopy (EIS), and adhesion tests.11-21 The corrosion extent of sa...
We report on the physicochemical properties and anti-corrosion performance of a non-chromated Zr/Zn conversion coating (NCP) on AA2024-T3. The immersion coating was formed on polished, degreased and deoxidized specimens. Electrochemical methods were used to assess the corrosion inhibition provided by the coating in laboratory tests. The results were compared with environmental exposure tests to assess the stand-alone corrosion protection. Coated AA6061-T6 and 7075-T6 specimens were also used in the environmental tests. Electrochemical testing in naturally-aerated 0.5 M Na 2 SO 4 + 0.1% NaCl revealed that the NCP coating shifted E corr positive by about 250 mV, suppressed anodic more than cathodic current around E corr by at least a factor of 10x and shifted E pit more noble. The coating functions more as an anodic inhibitor through barrier layer protection. The coating provided excellent corrosion protection to all three alloys during a 14-day full immersion test in 0.5 M Na 2 SO 4 + 0.1% NaCl. However during 14-day neutral salt spray and thin-layer mist tests, NCP failed to provide much stand-alone corrosion protection to the aluminum alloys and the anti-corrosion properties were found to be inferior to TCP conversion coatings of comparable thickness. A 7-day beach exposure revealed the NCP coating also provides little resistance to galvanic corrosion on the aluminum alloys as compared to TCP coatings. The results demonstrate that laboratory evaluation of the anti-corrosion properties of non-chromated conversion coatings does not always reflect coating performance during accelerated degradation or environmental exposure. The inferior anti-corrosion behavior of NCP, as compared to TCP, is due to (i) inherent defect density of the former (i.e., reduced throwing power) and ( AA2024 and AA7075 are high-strength aluminum alloys that derive their properties from their alloying components. They are used on civilian and military aircraft because of their light weight and mechanical strength. 1,2 However, many of the constituent particles, such as the Al 2 CuMg phase (so-called S-phase) in AA2024 3,4 and Mg(ZnCuAl) 2 in AA7075, 5-8 lead to corrosion challenges. More noble intermetallic particles play a critical role in the corrosion susceptibility of aluminum alloys as they can give rise to localized corrosion, such as pitting and exfoliation, because of the formation of galvanic cells with the surrounding aluminum. [9][10][11] The intermetallic phases tend to function as cathodic sites supporting oxygen reduction, which can drive the localized dissolution of nearby aluminum. The shape, size and chemical composition of the intermetallic particles are determined by the processing route (heat-treatment and forming) carried out on the aluminum alloy. [9][10][11] Multilayer coating systems (conversion coating + primer + topcoat) are required to protect aerospace aluminum alloys from corrosion in service. Traditional coating systems contain hexavalent chromium (Cr(VI)) in both the conversion coating and primer, volatile orga...
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