Abstract
Considering aviation and space sectors, aluminium alloys are commonly used due to its excellent mechanical and physical properties. Though satellite hard-ware is confined to controlled environment, it requires anticorrosive treatment over metal substrate followed by a systematic coating scheme. The trivalent chromium coating was deposited over three aluminium alloys namely AA6063, AA7075 and AA6082. The variation in corrosion resistance property of trivalent chromium over each aluminium alloy has been studied in detail. The Neutral Salt Spray (NSS) test result shows that trivalent chromium coating over AA7075 alloy is affected by pitting corrosion compared to other two alloys. In addition to that, NSS test also proves that thickness of the layer does not have any influence corrosion resistance property of trivalent chromium coating. Furthermore, ions in trivalent chromium coating was identified using Secondary Neutral Mass Spectroscopy (SNMS) and degradation of coating in a corrosive liquid studied using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) instrument to understand mechanism of corrosion. The results indicated that both coating and substrate is involved in corrosion process. The number of elements dissolved in to salt solution reveals the weak ionic bonding of coating towards substrates. In comparison, AA7075 alloy has weaker bonding than AA6082 and AA6063 series. The alloying elements such as zinc and copper are removed from substrate by corrosive solution.
Highlights
The Secondary Neutral Mass Spectroscopy (SNMS) mass spectroscopy analysis of trivalent chromium coating over three different materials as mention earlier shows significant proof that coating consist of similar species independent of substrate on which coating is formed
The mass spectrum graph from SNMS reveals that Zr, Cr, Al, F, O atoms are present inside coating that is in accordance with previous work by Manchester group [13] possibly due to usage of the same SurTec 650 solution as shown in Figure 2 In general, elemental analysis of coating can be made by XPS (X-ray Photoelectron Spectroscopy)
The following images are the samples of AA7075, AA6063 and AA6082 after salt spray test
Summary
Hexavalent chromium ions (Hex chrome or Cr6+) were employed under chromate conversion coating till date. Industry and research groups are focused towards replacement of traditional Cr6+ chromating [3] In this regard, trivalent chromium (Cr3+) ions seem to be the best alternative due to its low toxicity. Trivalent chromium ions exhibit good corrosion resistance in tropical environments because most of the launching base of Europe Space Agency (ESA) is located at high humid, high temperature and salty atmosphere [4]. This paper evaluates suitability of trivalent chromium coating in accordance to ASTM and ISO standards. In present study ICP-OES have been employed to analyze degradation of coated and non-coated aluminium alloy in 0.85 M of NaCl corrosive solution that is recommended for NSS test by ASTM B117 standard. The salt spray showed positive result only for aluminium that has low alloying elements in its elemental constituents
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
