Characterization of trivalent chromium process coating on AA2024-T3

Guo, Yang; Frankel, G. S.

doi:10.1016/j.surfcoat.2012.03.046

Cited by 122 publications

(182 citation statements)

References 19 publications

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“…7,9,10 In one study, it was proposed that a first, dense layer is formed by hydrolysis of Cr(OH) x (3-x)+ and Zr(OH) x (4-x)+ ions that retards oxidation of the alloy and reduction of protons; a second layer then forms by preferential deposition of zirconium hydroxide at transient cathodic sites.…”

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confidence: 99%

Formation of a Trivalent Chromium Conversion Coating on AA2024-T351 Alloy

Hashimoto

Walton

et al. 2015

J. Electrochem. Soc.

105

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The formation of a trivalent chromium conversion coating on AA2024-T351 aluminum alloy has been studied using electron microscopy, scanning Kelvin probe force microscopy, ion beam analysis and X-ray photoelectron spectroscopy (XPS). The coating contained oxide, hydroxide, fluoride and sulfate species, and consisted of two main layers: a zirconium-and chromium-rich outer layer and a thinner, aluminum-rich, inner layer. XPS indicated that zirconium and chromium are mainly present as ZrO 2 , Cr(OH) 3 , Cr 2 (SO 4 ) 3 and CrF 3 . In addition, a thin aluminum-rich layer, probably composed of hydrated alumina, occurred transiently at the coating surface. The coating above cathodic second phase particles was usually thicker than that above the matrix due to the locally increased alkalinity. In the early stages of treatment, the thickest coating formed above the S-phase particles. Localized corrosion and copper enrichment of the matrix occurred at the coating base. The localized corrosion is possibly related to the observed accumulation of fluoride ions at the inner layer and the enrichment of copper in the alloy. The thickness of the coating above the alloy matrix was significantly less than that of a coating formed on high purity aluminum. AA2024-T3 aluminum alloy is widely used in the aerospace industry due to its high strength to weight ratio and damage tolerance. However, the presence of copper as a primary alloying element leads to an increased susceptibility to localized corrosion, especially pitting corrosion.2,3 Hence, the alloy requires protective treatments to provide the required corrosion resistance for many applications. Such treatments often involve the use of chromate conversion coatings. However, the toxicity and high disposal costs of Cr(VI) wastes necessitate the development of eco-friendly alternatives. 4,5 One promising candidate treatment uses a trivalent chromium bath, generally consisting of zirconium hexafluoride and trivalent chromium salts. 6,7 The coating formation involves the dissolution of the native oxide on the aluminum surface due to the acidic, fluoride-containing solution, 8 and the subsequent pH-driven deposition of zirconium and chromium species. The increase in interfacial pH is promoted by the cathodic reactions, i.e. hydrogen evolution and/or oxygen reduction. 9Previous work has indicated that the resultant coating consists of two layers. 7,9,10 In one study, it was proposed that a first, dense layer is formed by hydrolysis of Cr(OH) x (3-x)+ and Zr(OH) x (4-x)+ ions that retards oxidation of the alloy and reduction of protons; a second layer then forms by preferential deposition of zirconium hydroxide at transient cathodic sites.10 Other studies revealed a coating composed of an outer, hydrated zirconium-rich layer that also contains chromium species and an inner, aluminum-rich layer. 7,9 Several investigations have been carried out to determine whether Cr(VI) species are formed in trivalent chromium conversion coatings. No Cr(VI) was detected by UV-visible spectroscopy in as-de...

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Formation of a Trivalent Chromium Conversion Coating on AA2024-T351 Alloy

Hashimoto

Walton

et al. 2015

J. Electrochem. Soc.

105

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“…4. Tieto výsledky sú v súlade s dosiaľ publikovanými zisteniami, pričom sa zistilo, že povlaky na báze zirkónia vykazujú vynikajúcu adhéziu, ale ich ochrana voči korózii je nízka, ak nie sú pokryté nátermi [4,5].…”

Section: Výsledky a Diskusiaunclassified

Influence of zirconium (IV) cations on sealing quality of anodized aluminium

Zemanová¹,

Bobok²,

Szelag

2014

Koroze a Ochrana Materialu

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1 Czech AssociaƟon of Corrosion Engineers ÚVODHliník je ľahký kov, ktorý je schopný v atmosfére sa oxidovať a vytvoriť prirodzenú ochrannú vrstvu oxidu hlinitého hrúbky niekoľko nanometrov. Na dosiahnutie väčšej hrúbky oxidu hlinitého sa používa elektrochemický proces anodickej oxidácie hliníka tzv. eloxovanie, pričom v závislosti od pracovných podmienok je možné dosiahnuť hrúbku oxidu hlinitého niekoľko desiatok mikrometrov. Použitý elektrolyt, čas procesu a použitý prúd resp. napätie sú rozhodujúcimi parametrami na docielenie požadovaných vlastností anodickou oxidáciou pripraveného oxidu hlinitého (anodický Al 2 O 3 ) [1]. Elektrochemicky získaný oxid hlinitý je vo svojej podstate pórovitý a nakoľko sa používa pre dekoratívne funkčné účely, z ktorých možno spomenúť automobilový, letecký a kozmický priemysel, je nevyhnutná jeho vysoká korózna odolnosť, ku ktorej prispieva uzavretie pórov anodického Al 2 O 3 označované aj utesnenie (sealing). Vysoko účinné utesnenie anodického Al 2 O 3 sa doposiaľ realizovalo na báze šesťmocného chrómu, ale smernica EU 2000/53/EC obmedzuje používanie šesťmocného chrómu a preto dochádza k systematickému hľadaniu náhrady tohto typu utesnenia. K najznámejším spôsobom utesnenia patrí hydrotermálne (ekologické a vysoko nákladné), s SiO 2 (problémy s odstraňovaním v odpadových vodách -ČOV) a na báze Ni, ktorého používanie do budúcna zostáva otázne kvôli jeho toxicite. Alternatívu predstavuje utesnenie na báze kovov vzácnych zemín a používanie prípravkov na báze prvkov 4. skupinyzirkónia, titánu [2]. Tieto procesy sú samy osebe rovnako vysoko nákladné, napriek tomu predstavujú alternatívu pre svoje výnimočné vlastnosti, ku ktorým patrí vysoká adhézia. Nevýhodou je, že tieto zlúčeniny sa používajú vo forme zlúčenín s fl uoridovými aniónmi. Zirkónium patrí ku kovom, ktoré sa v atmosfére a vhodných roztokoch pokrývajú pasivačnou vrstvou, má dobré mechanické vlastnosti, je netoxický, bioinert, avšak vo fyziologických roztokoch podlieha bodovej korózii. Vplyv zirkonièitých katiónov na kvalitu utesnenia anodicky oxidovaného hliníka Influence of zirconium (IV) cations on sealing quality of anodized aluminium

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“…[34][35][36][37][38][39] The trivalent conversion coating (TCC) typically contains fluoride salts (NaF), trivalent chromium salts (Cr(OH) 3 or Cr 2 (SO 4 ) 3 ) and hexafluorozirconate (ZrF 6 2− ). [40][41][42][43] Surface film formation takes place in an acidic environment where the co-precipitation of hydrated zirconia and trivalent chromium hydroxides occurs, hence providing high barrier properties. 40 However, due to the absence of hexavalent chromium within the coating, a significant decrease in the socalled self-healing properties associated with trivalent chromium protection strategies was noted.…”

Section: Introductionmentioning

confidence: 99%

Chromate replacement: what does the future hold?

Gharbi

Thomas

Smith³

et al. 2018

npj Mater Degrad

177

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The ubiquitous use of chromium and its derivatives as corrosion preventative compounds accelerated rapidly after the second industrial revolution, with such compounds now integral to modern society. However, the detrimental impact of chromium compounds on the environment and human health has prompted the need to revisit the majority of current industrial corrosion protection measures. This review retraces the origins of chromium replacement motivations, introducing the various legislative actions aimed at diminishing the use of chromium compounds, and critically reviews alternative corrosion preventative technologies developed in the recent decades to now. The review, herein, is intended for a broad audience in order to provide a concise update to an increasingly timely issue.

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Characterization of trivalent chromium process coating on AA2024-T3

Cited by 122 publications

References 19 publications

Formation of a Trivalent Chromium Conversion Coating on AA2024-T351 Alloy

Formation of a Trivalent Chromium Conversion Coating on AA2024-T351 Alloy

Influence of zirconium (IV) cations on sealing quality of anodized aluminium

Chromate replacement: what does the future hold?

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