Atmospheric corrosion of Mg and MgAl alloys – characterization and mechanisms
Doktorsavhandling, 2015

The atmospheric corrosion of Mg and MgAl alloys was investigated. Corrosion tests were performed in the laboratory under controlled environment. CP Mg, AM50 and AZ91 samples were exposed at 95% RH and 22°C in the presence and in the absence of 400 ppm CO2 for 24 h to 672 h. The passive film was characterized by means of XPS, FTIR, AES and XRD. It was shown that the film consisted MgO/Mg(OH)2 with carbonate on top of the film in the presence of CO2. In addition, Al3+ was present in the film formed on the alloys. Thickening of the surface film was described in terms of a hydration mechanism. Also, a dissolution-precipitation mechanism was proposed for the break-down of the passive film in humid air. FIB cross-section revealed that in the presence of CO2 dissolution of the metal substrate increased compared to CO2-free exposures. The NaCl–induced atmospheric corrosion of Mg and MgAl alloys was studied in the same environment. Brucite was the main corrosion product in the absence of CO2. In the case of the alloys, meixnerite was also detected. Magnesium hydroxy carbonates were the dominating corrosion products in the presence of 400 ppm CO2. All tested materials exhibited higher corrosion rates in the absence of CO2 compared to exposures with CO2. The electrochemical corrosion cells were more extensive in the absence of CO2 compared to when 400 ppm CO2 was present. EDX analysis revealed that η-phase particles were more efficient cathodes compared to the β-phase and the interdendritic areas. Using the FIB and BIB techniques combined with SEM-EDX it was shown that Cl accumulated at the bottom of the pits. The localized corrosion in the presence of NaCl (aq) and in the presence and absence of CO2 was investigated in 2D and 3D. Plan view characterization was performed by means of SE and BSE imaging in SEM. Interference microscopy was employed in order to study the distribution of the pits after corrosion product removal. SEM-3D imaging was performed using a FIB-SEM system investigating pitted regions. It was showed that the β-phase acted as a barrier against corrosion, especially in the case of alloy AZ91. In the absence of CO2 the corrosion pits tended to interconnect below the metal surface forming severely pitted regions. In the presence of CO2 the pits were isolated and shallower. The beneficial effect of CO2 on corrosion is attributed to its acidity that caused neutralization of the catholyte.

EDX

CO2

XPS

Mg and MgAl alloys

NaCl

Atmospheric corrosion

Characterization

3D imaging

SEM

BIB

Thin film

FTIR

XRD

Pitting

Interference microscopy

FIB

10:an, Förskarhus I, Kemihuset, Kemigården 4, Göteborg
Opponent: Prof. Raúl Arrabal

Författare

Mehrdad Shahabi Navid

Chalmers, Kemi och kemiteknik, Energi och material, Oorganisk miljökemi

Magnesium och magnesium-aluminiumlegeringar har rönt mycket uppmärksamhet under de senaste decennierna. Deras betydligt lätta kombinerat med god rumstemperatur styrka har gjort dem särskilt intressant för fordons- och flygindustrin. Magnesium är känslig för korrosion i synnerhet i närvaro av kloridinnehållande salter. När de utsätts för en atmosfär med viss fuktighet, magnesium och dess legeringar oxiderar. Sammansättningen av exponeringsatmosfär naturligtvis kan påverka mekanismen för korrosion. Sålunda är det viktigt att undersöka atmosfärisk korrosion av magnesium och dess legeringar under kontrollerad tillstånd.

Magnesium and magnesium-aluminum alloys have attracted a lot of attention in recent decades. Their significantly light weight combined with good room temperature strength has made them particularly interesting for automotive and aeronautical industries. Magnesium is susceptible to corrosion especially in the presence of chloride containing salts. When exposed to an atmosphere with certain humidity, magnesium and its alloys get oxidized. The composition of the exposure atmosphere of course can affect the mechanism of corrosion. Thus, it is important to investigate the atmospheric corrosion of magnesium and its alloys under a controlled condition.

Ämneskategorier

Materialteknik

Övrig annan teknik

Metallurgi och metalliska material

Korrosionsteknik

Styrkeområden

Materialvetenskap

ISBN

978-91-7597-249-7

10:an, Förskarhus I, Kemihuset, Kemigården 4, Göteborg

Opponent: Prof. Raúl Arrabal