A Study of the Localized Ceria Coating Deposition on Fe-Rich Intermetallics in an AlSiFe Cast Alloy

Abstract

Corrosion inhibiting conversion coating formation is triggered by the activity of micro-galvanic couples in the microstructure and subsequent local increase in pH at cathodic sites, which in the case of aluminium alloys are usually intermetallics. Ceria coatings are formed spontaneously upon immersion of aluminium alloys in a cerium conversion coating solution, the high pH gradient in the vicinity of intermetallics drives the local precipitation of ceria conversion compounds. Cu-rich intermetallics demonstrate a highly cathodic nature and have shown the local precipitation reaction to occur readily. Fe-rich intermetallics are, however, weaker cathodes and have shown varied extents of localized deposits and are in focus in the current work. Model cast Al-7wt.%Si alloys have been designed with 1 wt.% Fe, solidified at different cooling rates to achieve two different microstructures, with big and small intermetallics, respectively. Upon subjecting the two microstructures to the same conversion coating treatment (immersion in a 0.1 M CeCl3 solution) for a short period of 2 h, preferential heavy deposition on the boundaries of the big intermetallics and light deposition on the small intermetallics was observed. Based on these observations, a mechanism of localized coating initiation at these Fe-rich intermetallic particles (IM) is proposed.

Improvement in comprehensive properties of nanocomposite coatings electro-codeposited with nitride and ceria

“…A number of reinforcing particles were added into the nickel matrix to synthesise composite coatings with specific functions and applications. For instance, hard ceramic particles such as carbides [7,8], carbon nanotubes [9], oxides [10,11], borides [12], and nitrides [13] are selected as enhancing phases for MMCs to gain better mechanical and wear properties. Self-lubricating particles like graphene [14], sulphides [15,16], h-BN [17,18], etc.…”

The influence of cathodic θ-Al2Cu intermetallic geometry on the localized-corrosion-triggered conversion coating deposition

“…Cu is typically added to enhance the strength and hardness of aluminium 1 by solution and precipitation hardening While the microstructural heterogeneities benefit the alloy´s mechanical properties, they are detrimental to the corrosion resistance 2 and are the primary cause of localized corrosion in aluminium alloys 3 . Some intermetallic (IM) particles that contain Mg and Si are anodic relative to the matrix 4,5 , several of the other phases containing Cu, Fe or Mn are cathodic [6][7][8][9][10][11] .…”

Complementary corrosion protection of cast AlSi7Mg0.3 alloy using Zr-Cr conversion and polyacrylic/siloxane-silica multilayer coatings

“…Over the last few decades, many research studies in corrosion protection have focused on developing effective coatings to replace environmentally hazardous chromate conversion coatings containing chromium(VI) [5][6][7][8] . In the last twenty years, various environmentally acceptable and efficient corrosion protective conversion coating systems based on manganese 9,10 , titanium 11,12 , cerium [13][14][15][16] , trivalent chromium process (TCP) [17][18][19][20] , and zirconium 12,[21][22][23][24][25][26][27][28][29][30] have been investigated. The zirconiumbased coatings (ZrCCs) are already commercialised 12 .…”

“…Similar properties were noted for the commercial hexafluoro-zirconate trivalent chromium coating SurTec 650® in sodium chloride solution and simulated acid rain 24 . Compared to wrought aluminium alloys, relatively little is known about the electrochemical performance of conversion coatings on cast alloys 14,50 .…”