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Atmospheric corrosion of zinc-aluminum and copper-based alloys in chloride-rich environments: Microstructure, corrosion initiation, patina evolution and metal release
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Fundamental understanding of atmospheric corrosion mechanisms requires an in-depth understanding on the dynamic interaction between corrosive constituents and metal/alloy surfaces. This doctoral study comprises field and laboratory investigations that assess atmospheric corrosion and metal release processes for two different groups of alloys exposed in chloride-rich environments. These groups comprise two commercial Zn-Al alloy coatings on steel, Galfan™ (Zn5Al) and Galvalume™ (Zn55Al), and four copper-based alloys (Cu4Sn, Cu15Zn, Cu40Zn and Cu5Zn5Al). In-depth laboratory investigations were conducted to assess the role of chloride deposition and alloy microstructure on the initial corrosion mechanisms and subsequent corrosion product formation. Comparisons were made with long-term field exposures at unsheltered marine conditions in Brest, France.

A multitude of surface sensitive and non-destructive analytical methods were adopted for detailed in-situ and ex-situ analysis to assess corrosion product evolution scenarios for the Zn-Al and the Cu-based alloys. Scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS) were employed for morphological investigations and scanning Kelvin probe force microscopy (SKPFM) for nobility distribution measurements and to gain microstructural information. SEM/EDS, infrared reflection-absorption spectroscopy (IRAS), confocal Raman micro-spectroscopy (CRM) and grazing incidence x-ray diffraction (GIXRD) were utilized to gain information on corrosion product formation and possibly their lateral distribution upon field and laboratory exposures. The multi-analytical approach enabled the exploration of the interplay between the microstructure and corrosion initiation and corrosion product evolution.

A clear influence of the microstructure on the initial corrosion product formation was preferentially observed in the zinc-rich phase for both the Zn-Al and the Cu-Zn alloys, processes being triggered by microgalvanic effects. Similar corrosion products were identified upon laboratory exposures with chlorides for both the Zn-Al and the Cu-based alloys as observed after short and long term marine exposures at field conditions. For the Zn-Al alloys the sequence includes the initial formation of ZnO, ZnAl2O4 and/or Al2O3 and subsequent formation of Zn6Al2(OH)16CO3·4H2O, and Zn2Al(OH)6Cl·2H2O and/or Zn5(OH)8Cl2·H2O. The patina of Cu sheet consists of two main layers with Cu2O predominating in the inner layer and Cu2(OH)3Cl in the outer layer, and with a discontinuous presence of CuCl in-between. Additional patina constituents of the Cu-based alloys include SnO2, Zn5(OH)6(CO3)2, Zn6Al2(OH)16CO3·4H2O and Al2O3. General scenarios for the evolution of corrosion products are proposed as well as a corrosion product flaking mechanism for some of the Cu-based alloys upon exposure in chloride-rich atmospheres.

The tendency for corrosion product flaking was considerably more pronounced on Cu sheet and Cu4Sn compared with Cu15Zn and Cu5Al5Zn. This difference is explained by the initial formation of zinc- and zinc-aluminum hydroxycarbonates Zn5(OH)6(CO3)2 and Zn6Al2(OH)16CO3·4H2O on Cu15Zn and Cu5Al5Zn, corrosion products that delay the formation of CuCl, a precursor of Cu2(OH)3Cl. As a result, the observed volume expansion during transformation of CuCl to Cu2(OH)3Cl, and the concomitant flaking process of corrosion products, was less severe on Cu15Zn and Cu5Al5Zn compared with Cu and Cu4Sn in chloride-rich environments. The results confirm the barrier effect of poorly soluble zinc and zinc-aluminum hydroxycarbonates Zn5(OH)6(CO3)2 and Zn6Al2(OH)16CO3·4H2O, which results in a reduced interaction between chlorides and surfaces of Cu-based alloys, and thereby reduced formation rates of easily flaked off corrosion products. From this process also follows reduced metal release rates from the Zn-Al alloys.

Abstract [sv]

Bättre molekylär förståelse för metallers atmosfäriska korrosion kräver en fördjupad kunskap i det dynamiska samspelet mellan atmosfärens korrosiva beståndsdelar och metallytan. Denna doktorsavhandling omfattar laboratorie- och fältundersökningar av korrosions- och metallfrigöringsprocesser av två grupper av legeringar som exponerats i kloridrika atmosfärsmiljöer: två kommersiella Zn-Al beläggningar på stål, Galfan™ (Zn med 5% Al, förkortat Zn5Al) och Galvalume™ (Zn55Al), samt fyra kopparbaserade legeringar (Cu4Sn, Cu15Zn, Cu40Zn och Cu5Zn5Al). Undersökningar har genomförts i renodlade laboratorie-miljöer med för-deponerade NaCl-partiklar i en atmosfär av varierande relativ fuktighet. Syftet har varit att utvärdera betydelsen av kloriders deposition och legeringarnas mikrostruktur på korrosionsmekanismen samt bildandet av korrosionsprodukter. Jämförelser av korrosionsmekanismer har även gjorts efter flerårsexponeringar av samma legeringar i en marin fältmiljö i Brest, Frankrike.

Undersökningarna har baserats på ett brett spektrum av analysmetoder för detaljerade studier dels under pågående atmosfärisk korrosion (in-situ), och dels efter avslutad korrosion (ex-situ). Legeringarnas mikrostruktur och tillhörande variation i ädelhet hos olika faser har undersökts med svepelektronmikroskopi och energidispersiv röntgenmikroanalys (SEM/EDS) samt med en variant av atomkraftsmikroskopi (engelska: scanning Kelvin probe force microscopy, SKPFM). Korrosionsprodukternas tillväxt har analyserats in-situ med infraröd reflektions-absorptionsspektroskopi (IRAS), samt morfologi och sammansättning av bildade korrosionsprodukter ex-situ med SEM/EDS, konfokal Raman mikro-spektroskopi (CRM) samt röntgendiffraktion vid strykande ifall (GIXRD). Det multi-analytiska tillvägagångssättet har medfört att det komplexa samspelet mellan de skilda legeringarnas mikrostruktur, korrosionsinitiering och bildandet av korrosionsprodukter kunnat studeras i detalj.

En tydlig påverkan av mikrostruktur på det initiala korrosionsförloppet har kunnat påvisas. Korrosionsinitieringen sker företrädesvis i mer zinkrika faser för såväl Zn-Al- som Cu-Zn-legeringar och orsakas av mikro-galvaniska effekter mellan de mer zinkrika, mindre ädla, faserna och omgivande faser. Deponerade NaCl-partiklar påskyndar den lokala korrosionen oberoende av mikrostruktur. Snarlika sekvenser av korrosionsprodukter har kunnat påvisas såväl efter laboratorie- som fältexponeringar. För Zn-Al-legeringar bildas först ZnO, ZnAl2O4 och/eller Al2O3, därefter Zn6Al2(OH)16CO3·4H2O och Zn2Al(OH)6Cl·2H2O och/eller Zn5(OH)8Cl2·H2O. På ren koppar bildas ett inre skikt dominerat av Cu2O, ett mellanskikt av CuCl och ett yttre skikt med i huvudsak Cu2(OH)3Cl. Beroende på legeringstillsats har även SnO2 och Zn5(OH)6(CO3)2 kunnat identifieras.

En mekanism för flagning av korrosionsprodukter på kopparbaserade legeringar i kloridrika atmosfärer har utvecklats. Tendensen för flagning har visat sig vara mycket mer uttalad på ren Cu och Cu4Sn än på Cu15Zn och Cu5Al5Zn. Skillnaden kan förklaras med hjälp av det tidiga bildandet av Zn5(OH)6(CO3)2 och Zn6Al2(OH)16CO3·4H2O på Cu15Zn och Cu5Al5Zn som fördröjer bildandet av CuCl, en föregångare till Cu2(OH)3Cl. Därigenom hämmas även den observerade volymexpansionen som sker när CuCl omvandlas till Cu2(OH)3Cl, en process som visar sig vara den egentliga orsaken till att korrosionsprodukterna flagar. Resultaten bekräftar barriäreffekten hos de mer svårlösliga faserna Zn5(OH)6(CO3)2 och Zn6Al2(OH)16CO3·4H2O, vilken dels resulterar i en minskad växelverkan mellan klorider och de legeringsytor där dessa faser kan bildas, och dels i en reducerad metallfrigöringshastighet.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , xii, 74 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2014:27
Keyword [en]
atmospheric corrosion, chloride deposition, Zn-Al alloy coatings on steel microstructure, Cu sheet and Cu alloys, microstructure, corrosion initiation, corrosion product evolution, metal release, SEM, IRAS, CRM.
National Category
Natural Sciences
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-151180ISBN: 978-91-7595-203-1 (print)OAI: oai:DiVA.org:kth-151180DiVA: diva2:746954
Public defence
2014-09-26, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
Autocorr, RFSR-CT-2009-00015 Corrosion of heterogeneous metal-metal assemblies in the automotive industryAtmospheric corrosion and environmental metal dispersion from outdoor construction materials
Note

QC 20140915

Available from: 2014-09-15 Created: 2014-09-15 Last updated: 2014-09-15Bibliographically approved
List of papers
1. The initial release of zinc and aluminum from non-treated Galvalume and the formation of corrosion products in chloride containing media
Open this publication in new window or tab >>The initial release of zinc and aluminum from non-treated Galvalume and the formation of corrosion products in chloride containing media
Show others...
2011 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 258, no 10, 4351-4359 p.Article in journal (Refereed) Published
Abstract [en]

This study explores the initial release of zinc and aluminum from non-treated Galvalume and the parallel formation of corrosion products when exposed to synthetic seawater and rainwater of different chloride content. Comparisons were made with long-term field exposures at non-sheltered marine conditions. Observed release rates from short-term conditions agree qualitatively with the long-term findings with a selective release of zinc over aluminum. The release and corrosion processes were intertwined through the formation of corrosion products with properties that influence the long-term release process. Prior to exposure, Al2O3 dominated the entire surface, and was subject to local destruction upon interaction with chloride ions. As a consequence Al2O3 was gradually replaced and covered by zinc-rich corrosion products primarily in interdendritic areas during the first year of marine exposure. This was followed by the gradual formation and integration of aluminum-rich corrosion products, reflected by an increased zinc release rate during the first year, followed by a gradually decreased rate during subsequent years. The importance of Al2O3 was also evident in deaerated synthetic rainwater or seawater, where the formation of Al2O3 was presumably hindered. In synthetic rain water this resulted in a higher ratio between released aluminum and zinc compared with non-deaerated conditions.

Keyword
Galvalume, Metal release, Corrosion products, Chlorides
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-81529 (URN)10.1016/j.apsusc.2011.12.112 (DOI)000300991400022 ()2-s2.0-84862826864 (Scopus ID)
Funder
Knut and Alice Wallenberg Foundation
Note
QC 20120402Available from: 2012-02-10 Created: 2012-02-10 Last updated: 2017-12-07Bibliographically approved
2. Atmospheric corrosion of Galfan coatings on steel in chloride-rich environments
Open this publication in new window or tab >>Atmospheric corrosion of Galfan coatings on steel in chloride-rich environments
2013 (English)In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 73, 62-71 p.Article in journal (Refereed) Published
Abstract [en]

Galfan coatings on steel in laboratory exposures with predeposited NaCl and cyclic wet/dry conditions exhibit nearly the same corrosion products as after 5 years of marine exposure. A general scenario for corrosion product evolution on Galfan in chloride-rich atmospheres is proposed. It includes the initial formation of ZnO, ZnAl2O4 and Al2O3 and subsequent formation of Zn6Al2(OH)(16)CO3 center dot 4H(2)O, and Zn2Al(OH)(6)Cl center dot 2H(2)O and/or Zn5Cl2(OH)(8)center dot H2O. An important phase is Zn6Al2(OH)(16)CO3 center dot 4H(2)O, which largely governs the reduced long-term zinc runoff from Galfan. A clear influence of microstructure could be observed on corrosion initiation in the slightly zinc-richer eta-Zn phase adjacent to the beta-Al phase.

Keyword
Metal coatings, Zinc, Aluminum, IR spectroscopy, SEM, Atmospheric, corrosion
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-125546 (URN)10.1016/j.corsci.2013.03.025 (DOI)000320736900007 ()2-s2.0-84878375987 (Scopus ID)
Note

QC 20130813

Available from: 2013-08-13 Created: 2013-08-09 Last updated: 2017-12-06Bibliographically approved
3. Selected area visualization by FIB-milling for corrosion-microstructure analysis with submicron resolution
Open this publication in new window or tab >>Selected area visualization by FIB-milling for corrosion-microstructure analysis with submicron resolution
2013 (English)In: Materials letters (General ed.), ISSN 0167-577X, E-ISSN 1873-4979, Vol. 98, 230-233 p.Article in journal (Refereed) Published
Abstract [en]

We report the successful use of focussed ion beam (FIB) milling of trenches in a material of complex microstructure in order to visualize a selected area (32×32 μm) for further multi-analysis with submicron resolution. This capability is demonstrated for a Zn-5 wt% Al coating Galfan™ on steel. The very same eutectic surface area was analyzed by three complementary and independent techniques providing consistent information on the lateral distribution of morphology and elemental composition (scanning electron microscopy with x-ray microanalysis, SEM/EDS), topography and Volta potential (scanning Kelvin probe force microscopy, SKPFM) and oxide composition (confocal Raman microspectroscopy, CRM). The approach enables a straightforward way to explore the interplay between microstructure and local corrosion of metallic materials.

Keyword
Ion beam technology, Local corrosion, Local probing techniques, Metals and alloys, Microstructure, Complex microstructures, Confocal Raman microspectroscopy, Focussed ion beam millings, Ion beam technologies, Local probing, Scanning Kelvin probe force microscopy, Aluminum coatings, Corrosion, Ion beams, Milling (machining), Scanning electron microscopy, Surface topography
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-134666 (URN)10.1016/j.matlet.2013.02.027 (DOI)000320078300061 ()2-s2.0-84875137227 (Scopus ID)
Note

QC 20131210

Available from: 2013-12-10 Created: 2013-11-27 Last updated: 2017-12-06Bibliographically approved
4. Corrosion and runoff rates of Cu and three Cu-alloys in marine environments with increasing chloride deposition rate
Open this publication in new window or tab >>Corrosion and runoff rates of Cu and three Cu-alloys in marine environments with increasing chloride deposition rate
Show others...
2014 (English)In: Science of the Total Environment, ISSN 0048-9697, E-ISSN 1879-1026, Vol. 472, 681-694 p.Article in journal (Refereed) Published
Abstract [en]

Bare copper sheet and three commercial Cu-based alloys, Cu15Zn, Cu4Sn and Cu5Al5Zn, have been exposed to four test sites in Brest, France, with strongly varying chloride deposition rates. The corrosion rates of all four materials decrease continuously with distance from the coast, i.e. with decreasing chloride load, and in the following order: Cu4Sn > Cu sheet > Cu15Zn > Cu5Al5Zn. The patina on all materials was composed of two main layers, Cu2O as the inner layer and Cu-2(OH)(3)Cl as the outer layer, and with a discontinuous presence of CuCl in between. Additional minor patina constituents are SnO2 (Cu4Sn), Zn-5(OH)(6)(CO3)(2) (Cu15Zn and Cu5Al5Zn) and Zn6Al2(OH)(16)CO3 center dot 4H(2)O/Zn2Al(OH)(6)Cl center dot 2H(2)O/Zn5Cl2(OH)8 center dot H2O and Al2O3 (Cu5Al5Zn). The observed Zn- and Zn/Al-containing corrosion products might be important factors for the lower sensitivity of Cu15Zn and Cu5Al5Zn against chloride-induced atmospheric corrosion compared with Cu sheet and Cu4Sn. Decreasing corrosion rates with exposure time were observed for all materials and chloride loads and attributed to an improved adherence with time of the outer patina to the underlying inner oxide. Flaking of the outer patina layer was mainly observed on Cu4Sn and Cu sheet and associated with the gradual transformation of CuCl to Cu-2(OH)(3)Cl of larger volume. After three years only Cu5Al5Zn remains lustrous because of a patina compared with the other materials that appeared brownish-reddish. Significantly lower release rates of metals compared with corresponding corrosion rates were observed for all materials. Very similar release rates of copper from all four materials were observed during the fifth year of marine exposure due to an outer surface patina that with time revealed similar constituents and solubility properties.

Keyword
Atmospheric corrosion, Chloride deposition, Corrosion rates, Patina evolution, Metal release, Copper alloys
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-143709 (URN)10.1016/j.scitotenv.2013.11.080 (DOI)000331916100075 ()2-s2.0-84890091085 (Scopus ID)
Note

QC 20140331

Available from: 2014-03-31 Created: 2014-03-27 Last updated: 2017-12-05Bibliographically approved
5. Mechanistic studies of corrosion product flaking on copper and copper-based alloys in marine environments
Open this publication in new window or tab >>Mechanistic studies of corrosion product flaking on copper and copper-based alloys in marine environments
2014 (English)In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 85, 15-25 p.Article in journal (Refereed) Published
Abstract [en]

The mechanism of corrosion product flaking on bare copper sheet and three copper-based alloys in chloride rich environments has been explored through field and laboratory exposures. The tendency for flaking is much more pronounced on Cu and Cu-4 wt%Sn than on Cu-15 wt%Zn and Cu-5 wt%Al-5 wt%Zn. This difference is explained by the initial formation of zinc and zinc-aluminum hydroxycarbonates on Cu15Zn and Cu5Al5Zn, which delays the formation of CuCl, a precursor of Cu-2(OH)(3)Cl. As a result, the observed volume expansion during transformation of CuCl to Cu-2(OH)(3)Cl, and concomitant corrosion product flaking, is less severe on Cu15Zn and Cu5Al5Zn than on Cu and Cu4Sn.

Keyword
Alloy, Copper, IR spectroscopy, Raman spectroscopy, SEM, Atmospheric corrosion
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-148339 (URN)10.1016/j.corsci.2014.03.028 (DOI)000338388100003 ()2-s2.0-84901590805 (Scopus ID)
Note

QC 20140805

Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2017-12-05Bibliographically approved

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