J. Kruger

Stress corrosion cracking of rapidly solidified magnesium-aluminum alloys

Stress corrosion cracking (SCC) of rapidly solidified magnesium-aluminum alloys in aqueous solutions of potassium chromate and sodium chloride was investigated using electrochemical techniques, constant displacement rate tests, and optical and electron microscopy. Microcrystalline alloys containing 1 and 9 wt% aluminum were prepared using a melt-spinning process which yields continuous ribbons 15–25 μm thick. Potential-pulse and scratched electrode experiments showed that repassivation kinetics are improved both by rapid solidification and increased aluminum content. The melt-spun alloys experienced relatively uniform attack, and repassivated more rapidly and more completely than their as-cast counterparts. Both failed by transgranular stress corrosion cracking (TGSCC) in aqueous 0.21 M K2 CrO4 containing 0.6M NaCl at displacement rates between 5 × 10−5 and 9 × 10−3 mm s−1. In 0.6M NaCl, TGSCC occurred only near 3.6 × 10−3 mm s−1, while no stress corrosion was observed in chromate solution without chloride. Constant displacement rate tests in air after pre-exposure to the electrolyte indicated that TGSCC probably results from a hydrogen embrittlement process. Using reasonable estimates of the diffusivity of hydrogen in magnesium, analysis of the constant displacement rate and potential-pulse tests for Mg-9Al supports a model involving the formation of magnesium hydride ahead of the crack tip.

The nature of the passive film on iron and ferrous alloys

Abstract A review is given of experiments that have contributed to a better understanding of the nature of the passive film on iron and ferrous alloys. The important questions relating to the nature of passive films on iron that are considered are: (1) the thickness —are the films two- or three-dimensional? (2) the composition —do the films contain hydrogen and cations introduced by alloying elements or inhibitors? (3) the structure —are the films crystalline spinel structures or non-crystalline glassy structures? and (4) the electronic properties —are the films insulators or semiconductors?

Dynamic imaging microellipsometry: theory, system design, and feasibility demonstration

Dynamic imaging microellipsometry (DIM) is a new rapid full-field imaging technique for high spatial resolution studies of thin films. The DIM concept is based on radiometric polarizer, compensator, specimen, and analyzer ellipsometry combined with video and image processing techniques. The theoretical basis for this approach is developed using the Jones vector and matrix formalisms. Basic systems design is presented with error model predictions of ellipsometric accuracies better than 0.1 degrees for full-field Delta and psi images captured in a few seconds with spatial resolution under 10 microm. Initial feasibility tests have demonstrated interframe discriminations of 0.36 degrees for Delta and 0.082 degrees for psi.

Repassivation of Rapidly Solidified Magnesium‐Aluminum Alloys

Electrochemical techniques and electron microscopy were used to investigate the effects of rapid solidification processing (RSP) and aluminum content on the repassivation of binary magnesium-aluminum alloys in aqueous solutions of potassium chromate and sodium chloride (pH 9.1). This electrolyte resembles environments causing pitting corrosion and stress corrosion cracking in Mg-Al alloys, where chromate ions, applied as a corrosion protection measure, coexist with chloride ions indigenous to sea water and other natural electrolytes

Passivity of Fe in Anhydrous Propylene Carbonate

The passivation behavior of Fe in anhydrous propylene carbonate (PC) containing 0.5M LiCLO 4 was studied. The effect of small additions on the passivation observed was also studied

The use of x-ray absorption spectroscopic techniques to study the influence of alloying elements on passive films: For: Proc. 6th Int. Symp. on Passivity, Sapporo Japan

X-ray absorption spectroscopic techniques can be used to investigate the structure, composition, and electronic properties of in situ and ex-situ passive films on metal surfaces. Earlier EXAFS (extended x-ray absorption fine structure) studies on the effect of Cr on the structure of the passive film on iron indicated that the addition of Cr produces a less crystalline film than that which forms on pure iron. EXAFS by means of surface reflection (reflEXAFS) enables the study of surface films directly on bulk alloy surfaces. ReflEXAFS has been applied to a study of the films on rapidly solidified Mg alloys, in which it was also found that the more protective films have a less well ordered structure. In addition, the electronic properties of passive layers on FeCr alloys have been studied by means of reflEXAFS in which the NEXAFS (near edge x-ray absorption fine structure) was used to examine the effect of Cr and Cl− (introduced to initiate breakdown) on ex-situ passive films.

The Passivity of 304 Stainless Steel in Propylene Carbonate Solutions

This paper reports that the passivation behavior of 304 stainless steel in anhydrous propylene carbonate (PC) containing 0.5M LiAsF{sub 6} or 0.5M LiClO{sub 4} was studied. The air-formed film on 304SS is stable up to the oxidation potential of PC (PC{sub ox}). Scratch tests show that the bared 304SS surface repassivates in the anhydrous PC solutions of either electrolyte by chemisorption of PC molecules below PC{sub ox}. In PC/0.5M LiAsF{sub 6} solutions, the 304SS is not passivated at potentials above PC{sub ox}. This is attributed to the formation of a thin metastable perchlorate salt film or an adsorbed layer of perchlorate anions. When the perchlorate anions oxidize, the passivation becomes unstable and pitting occurs. Small (3-8 volume percent) additions of PC/0.5M LiClO{sub 4} to PC/0.5M LiAsF{sub 6} solutions raised the passive range to the perchlorate oxidation potential. Small quantities of water, propylene glycol, and propylene oxide added to PC lightly improve the passive range of the 304 stainless steel.

The passivity of iron and carbon steel in anhydrous propylene carbonate solutions

Abstract The passivation behavior of high purity iron and 1018 carbon steel in anhydrous propylene carbonate (PC) containing 0.5 M LiClO 4 or 0.5 M LiAsF 6 was studied. The air-formed film on Fe was stable and protective over a wide applied potential range in perchlorate solutions. Scratch tests showed that both bare Fe and steel passivate in the anhydrous PC solutions of either electrolyte by chemisorption of PC molecules below the oxidation potential of PC (PC ox ). At higher potentials, bare Fe passivates in PC/LiClO 4 via the formation of a Fe(ClO 4 ) 2 salt film. The air-formed film on carbon steel was protective up to the PC ox potential in either electrolyte. Above PC ox , deterioration of the air-formed film on steel was associated with sulfide surface inclusions. Pits initiated and grew at these inclusion sites. Scratch tests showed slower repassivation of the bare steel by the Fe(ClO 4 ) 2 salt film than on iron. Sulfides impeded this repassivation. Above PC ox , neither the iron or the steel are passive in PC/LiAsF 6 solutions. The higher solubility of the Fe(AsF 6 ) 2 salt prevents precipitation of a protective salt film.

The passivity of iron in mixtures of propylene carbonate and water

The passivation of Fe in binary mixtures of propylene carbonate and water containing 0.5 M LiClO4 has been investigated over the entire range of water-PC composition. While Fe was found to passivate in both totally aqueous and anhydrous solutions, over much of the composition range in between an unstable passivity was observed which led to localized attack of the surface. A passive film formed in aqueous solution was found to be stable upon exposure to the mixed solutions, though a removal of this film by cathodic reduction created bare surface sites which, when polarized to the original passive film formation potential, suffered from a pitting attack. These observations are explained in terms of the competition between the different passivation mechanisms for the organic and aqueous cases leading to localized metal loss because neither passivation mechanism can dominate in the solution mixtures.

The effects of water on the passive behavior of 1018 carbon steel in organic solutions

The passivation and breakdown behavior of 1018 carbon steel in propylene carbonate (PC) or dimethoxyethane (DME) mixtures with water and containing 0.5M LiAsF[sub 6] were studied. The behavior of the steel in the organic solvent/water mixtures was highly dependent on the organic solvent. The anodic polarization of carbon steel displayed active-passive behavior in 10--90 mole percent (m/o) PC/H[sub 2]O mixtures and a tenuous degree of stability within the passive range. The anodic polarization of carbon steel displayed no active-passive behavior in 50--90 m/o DME/H[sub 2]O mixtures and displayed active-passive behavior in 10--30 m/o DME/H[sub 2]O mixtures. The steel was stable within the passive range of these DME/H[sub 2]O solutions. The breakdown potential of the steel in DME/H[sub 2]O mixtures is more electropositive than the oxidation potential of the DME solvent at all molar ratios.