Analysis of the Gases Evolved during the Pitting Corrosion of Aluminum in Various Electrolytes

Abstract

This paper contains a preliminary report of the composition of the gases evolved during the pitting corrosion of aluminum in aqueous electrolytes containing respectively KCI, KSCN, or NaNO.. We believe the rather surprising resul~s will bear positively on understanding pitting mechanisms and reactions. First, each electrolyte has a unique set of gases. Second, using a light microscope (I, 2), it was observed in each system that the gas evolution begins when the potential is increased above the pitting potential, E_, and ceases when the potential is decreased b~low ~il E has been determined to be about -0.8 V pBtentials are referred to the saturated calomel electrode) for 1M CI-electrolytes by many investigators (e.g., 3-5). Galvele, et al have published pitting potentials of about 1.46 V and 0.96 V for 1M NaNO 3 and 1M KSCN, respectively (6, 7). Kaesche has observed that gas evolution occurs only above EQ for chloride electrolyte (8). Bargeron and Givens have observed blister formation, which requires gas production at the oxide-metal interface, as a step in the breakdown of the passivating oxide in chloride solutions (I, 2). As will be shown, the gases evolved from the NO~ and SCN-electrolytes can only be expJained by direct reaction of the anion with the metal. These results may also imply that CI-reacts directly with the metal.The aluminum used for observations under the light microscope was 0.25 mm thick discs of purity 99.999 percent. The discs were placed in an electrode holder as described previously (I, 2). For the gas collection, aluminum of purity 99.99 percent was machined into a cylinder about 1 cm high and 1 cm in diameter. Samples were degreased in acetone and pickled briefly in 1N NaOH. The samples used in the gas analysis experiments were screwed onto a Teflon holder which sealed the electrical connections from the electrolyte. Deionized water of resistivity 18 M~-cm and A.C.S. reagents (highest purity available) were used to prepare electrolytes. The electrolytes were deaerated with high purity argon before gas collection. The gas collection apparatus (a glass tube with a bellshaped piece on the end) was evacuated and filled with electrolyte. The evolved gases displaced the electrolyte in the tube, and I-2 cm 3 of gas was collected. Before entering the gas analyzer (VEECO Model SPI-IO), the gases passed through a trap at -50~ (alcohol and dry ice) to remove water. After an experiment the trap was warmed to room temperature, and gases from it were analyzed. Only water was detected from the trap.The sample potential was potentiostatically controlled. The reference electrode was a saturated calomel electrode connected by a bridge of the electrolyte through a Luggin capillary placed about 1 mm from the sample surface (the O.D. of the capillary was also about 1 mm).The mass spectrum of the gas collected from pitting aluminum at a potential of 1.5 V in 1M KCI is shown in Figure la. The spectrum shows the presence of H^ and the deaera-, . Z t tion gas Ar whlch is always present (Ar at m/e = 40 and Ar ++...

A Review of Trends for Corrosion Loss and Pit Depth in Longer-Term Exposures

“…It also will be the main barrier in the early part of Mode 2, but now for the corrosion rate controlled by outward hydrogen transportation. The latter is consistent with experimental observations (e.g., [57]). A mechanism for this has been proposed [56].…”

The contribution of hydrogen evolution processes during corrosion of aluminium and aluminium alloys investigated by potentiodynamic polarisation coupled with real-time hydrogen measurement

“…Thus, in many cases of practical interest, the corrosion rate of magnesium correlates well with the amount of hydrogen evolved. [2][3][4][5][6][7][8][9] For aluminium, hydrogen evolution can be triggered in specific environments typical of active corrosion sites such as, for example, pits or crevices, [10][11][12][13][14][15][16][17][18][19][20] increasing substantially the local corrosion rate. Besides corrosion, unwanted hydrogen evolution can be an important issue for other practical applications; for example, it is responsible for the reduction of the faradic efficiency of primary batteries and sacrificial anodes.…”

“…In some conditions, when the film is locally disrupted, such as for example during anodic polarisation in chloride-containing environment or at active corrosion sites (pits) during free corrosion, hydrogen evolution might occur locally, providing additional current sustaining corrosion propagation. 12,28 The increase in hydrogen evolution rate during anodic polarisation, which is observed both for aluminium [14][15][16][17][18][19] and magnesium, 3,4,29,30 is particularly evident and it has attracted significant interest, since it is contrary to what electrochemical theory would predict. The phenomenon is often referred to as 'negative difference effect' or 'superfluous' hydrogen evolution.…”

“…In the case of aluminium, it appears that there is a correlation between the occurrence of superfluous hydrogen and the onset of pitting during polarisation or even during free corrosion. [14][15][16][17][18][19]31 For example, Frankel observed abundant hydrogen evolution during the growth of 2-dimensional pits on aluminium 17 under anodic polarisation and estimated that the current associated with hydrogen evolution was approximately constant, regardless of the applied anodic potential. McCafferty 31 proposed a sequence of steps for the initiation and propagation of pitting on aluminium in the presence of chloride ions, and suggested that the formation of hydrogen gas might play a role in disrupting the passive film, by forming oxide blisters that rupture due to the high hydrogen pressure.…”

Development of an Electrochemical Balance to Measure Quantitatively Hydrogen Generation during Electrochemical Processes

“…It can be a side reaction during metal electrodeposition, 1-3 a parasitic reaction for batteries [4][5][6] and sacrificial anodes, [7][8][9][10] or the cathodic reaction for corrosion of metals with low electrochemical potential, such as for example magnesium [11][12][13][14][15][16][17][18][19][20][21][22][23] or aluminum. [24][25][26][27][28][29][30][31][32][33][34] The accurate measurement of the amount of hydrogen gas generated during an electrochemical process is important both to aid fundamental understanding and to optimize technological processes. The present work has originated from the authors' activity focusing on the measurement of the hydrogen evolved from magnesium during corrosion in aqueous environments; however the method presented here could be applied to virtually any study where quantitative measurement of gas evolving from an immersed electrode is required.…”