Quantitative Assessment of Localized Corrosion Occurring on Galvanized Steel Samples Using the Scanning Vibrating Electrode Technique

CORROSION ◽  
2004 ◽  
Vol 60 (5) ◽  
pp. 437-447 ◽  
Author(s):  
D. A. Worsley ◽  
H. N. McMurray ◽  
J. H. Sullivan ◽  
I. P. Williams
Keyword(s):  
Inductively Coupled Plasma ◽  
Metal Surfaces ◽  
Galvanized Steel ◽  
Localized Corrosion ◽  
Zinc Alloy ◽  
External Exposure ◽  
Pure Zinc ◽  
Average Mass ◽  
Faraday’S Law ◽  
Electrode Technique

Abstract Zinc and zinc alloy galvanized steel is used increasingly for structural cladding, automotive, and domestic appliance applications. In assessing the different galvanizing coatings, it is important to understand the nature of corrosion reactions occurring on the metal surfaces. To this end, the scanning vibrating electrode technique (SVET) has been used to study the effect of variation in metallic coating on the localization and intensity of corrosion reactions occurring on the bare metal surfaces when immersed in aerated 0.1% sodium chloride (NaCl). The samples used comprised pure zinc and galvanized steel substrates, namely electro-zinc (EZ), hot dip galvanized steel (HDG), iron (9%) zinc intermetallic (IZ), 5% aluminum zinc alloy (Galfan), and 55% aluminum zinc alloy (Zalutite). The SVET has the resolution and sensitivity to enable the number and intensity of active anodes to be quantified. Zinc galvanized materials show anodes, which do not deactivate within the 24 h of the test whereas zinc aluminum alloy anodes display typical anode lifetimes of 6 h to 12 h. The SVET data has been calibrated and integrated to provide a total current per scan and subsequently converted to zinc loss using Faraday's law. The total average mass losses obtained from 10-mm by 10-mm exposed areas were measured using the SVET: 1.133, 0.601, 0.432, 0.615, 0.264, and 0.051 mg for zinc, EZ, HDG, IZ, Galfan, and Zalutite, respectively, and these values were confirmed using inductively coupled plasma mass spectrometry (ICP-MS). The SVET data for zinc loss obtained over 24 h has been compared to external weathering data obtained after 2, 6, and 12 months of external exposure. There is an excellent correlation between metal runoff in initial external exposure and 24-h SVET experiments. In longer-term exposure, however, the IZ coating becomes covered in a metal hydr(oxide) layer, reducing runoff, and penetrative defects to the iron substrate in EZ lead to elevated runoff rates within 12 months.

Localized Corrosion of Binary Mg-Nd Alloys in Chloride-Containing Electrolyte Using a Scanning Vibrating Electrode Technique

CORROSION ◽  
2012 ◽  
Vol 68 (6) ◽  
pp. 489-498 ◽  
Author(s):  
G. Williams ◽  
K. Gusieva ◽  
N. Birbilis
Keyword(s):  
Localized Corrosion ◽  
Radial Expansion ◽  
Alloying Additions ◽  
Corrosion Rates ◽  
Surface Breakdown ◽  
Density Values ◽  
Intact Surface ◽  
Mean Current ◽  
Electrode Technique

The influence of neodymium (Nd) alloying additions in the 0.47 wt% to 3.53 wt% range on the localized corrosion behavior of Mg, when freely corroding in aqueous sodium chloride (NaCl) electrolyte, is investigated using an in situ scanning vibrating electrode technique (SVET). For all samples, the point of surface breakdown is an intense focal anode that expands radially with respect to time, revealing a cathodically activated interior, which is galvanically coupled with the local anode at the perimeter. However, for Nd compositions of ≤0.74%, radial expansion ceases within ca. 2 h of initiation, whereupon dark filiform-like corrosion features are observed, which traverse over the exposed Mg surface. For Nd additions of ≥1.25%, the radial expansion continues with time up to a point where the entire intact surface becomes consumed. The intensity of the local anode ring of circular corroded regions is seen to increase as more cathodically activated corroded surface becomes exposed. Mean current density values measured within these corroded areas increase progressively with Nd content, leading to a progressive rise in localized corrosion rates. The cathodic activation of corroded regions is proposed to derive from an enrichment of noble, Nd-rich intermetallic grains caused as the alpha-Mg phase becomes attacked at local anode sites.

Effect of pH on the Grain Size Dependence of Magnesium Corrosion

CORROSION ◽  
2012 ◽  
Vol 68 (6) ◽  
pp. 507-517 ◽  
Author(s):  
K. D. Ralston ◽  
G. Williams ◽  
N. Birbilis
Keyword(s):  
Grain Size ◽  
Corrosion Rate ◽  
Anodic Polarization ◽  
Localized Corrosion ◽  
Anodic Reaction ◽  
Left Behind ◽  
Kinetics Of ◽  
Electrode Technique

Prior works show that grain size can play a role in the corrosion of a metal; however, such works are nominally executed in a single electrolyte/environment at a single pH. In this work, the anodic and cathodic reaction kinetics of pure Mg specimens with grain sizes ranging from approximately 8 μm to 590 μm were compared as a function of pH in 0.1 mol dm−3 sodium chloride (NaCl) electrolytes using anodic polarization experiments and an in situ scanning vibrating electrode technique (SVET). Anodic polarization experiments showed that grain size is important in determining overall electrochemical response, but the environment dictates the form of the grain size vs. corrosion rate relationship (i.e., pH is the overall controlling factor). Consequently, the role of grain size upon corrosion cannot be fully assessed unless a variation in environment is simultaneously studied. For example, the anodic reaction, which dictates active corrosion, also dictates passivation, so the corrosion rate vs. grain size relationship has been shown to “flip” depending on pH. Further, SVET analysis of unpolarized Mg immersed in 0.1 mol dm−3 NaCl electrolyte at neutral pH showed that breakdown of passivity of cast Mg occurred after ~1 h immersion, giving filiform-like corrosion tracks. The front edges of these corrosion features were revealed as intense local anodes, while the remainder of the dark-corroded Mg surface, left behind as the anodes traversed the surface, became cathodically activated. In contrast, grain-refined Mg samples were significantly less susceptible to localized corrosion, and breakdown was not observed for immersion periods of up to 24 h.

Applications of the scanning reference electrode technique to localized corrosion

1993 ◽  
Vol 35 (1-4) ◽  
pp. 127-134 ◽  
Author(s):  
K.R. Trethewey ◽  
D.A. Sargeant ◽  
D.J. Marsh ◽  
A.A. Tamimi
Keyword(s):  
Reference Electrode ◽  
Localized Corrosion ◽  
Electrode Technique

Joint Interface Morphology of Friction Stir Spot Welded Aluminum Alloy Sheets and Plated Steel Sheets

2010 ◽  
Vol 654-656 ◽  
pp. 970-973 ◽  
Author(s):  
Keyan Feng ◽  
Mitsuhiro Watanabe ◽  
Shinji Kumai
Keyword(s):  
Aluminum Alloy ◽  
Melting Temperature ◽  
Steel Sheet ◽  
Alloy Sheet ◽  
Zinc Alloy ◽  
Joint Interface ◽  
Aluminum Alloy Sheet ◽  
Pure Zinc ◽  
Friction Stir ◽  
Steel Sheets

Friction stir spot welding (FSSW) was applied to lap joining of aluminum alloy sheets and steel sheets. A 1.2 mm-thick non-plated carbon steel sheet and plated steel sheets with zinc alloy (ZAM), pure zinc (GI), zinc alloy including Fe (GA) and Al-Si alloy (AS) were prepared. The melting temperature of the plated layer is 330, 420, 880 and 640°C, respectively. A 1.1 mm-thick 6022 aluminum alloy sheet was overlapped on the steel sheet. A rotating tool was inserted from the aluminum alloy sheet side and the probe tip was kept at the position of 0.2 mm above the lapped interface for 3 seconds. For ZAM and GI, original plated layers were removed from the interface and intermediate layers were formed at the joint interface. This is because the melting temperature of the plated layer was lower than the interface temperature under the rotating probe tip during the FSSW. In contrast to that, the partial original plated layer remained after welding, and additional layer formed at the plated layer /aluminum alloy interface for GA. For AS, Al-Fe intermetallic compound layer, which was formed at the original Al-Si alloy plated steel surface remained.

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