Anisotropy of nickel release and corrosion in austenitic stainless steels.

The study of 316L-type stainless steel reveals a significant anisotropy of nickel release that is dependent on the orientation of the test surface with respect to the casting and rolling direction. Cross-sectional specimens (transversal cuts with respect to the rolling direction) show a substantially higher sensitivity to corrosion phenomena compared with longitudinal cuts and they release nickel ions at rates 10-100 times higher. These findings indicate that orientation needs to be taken into account when interpreting test results, in particular when comparing different grades of austenitic stainless steel, as well as in product and production design.

Comparative corrosion study of "Ni-free" austenitic stainless steels in view of medical applications.

The role of nickel in the biological response to alloys used in medical devices is of immense significance with regard to toxicology and biological performance. There is now a tendency to take nickel out of alloys for medical applications. However, this needs careful evaluation since no compromise is acceptable with regard to mechanical properties, corrosion resistance or any other harmful consequences due to the nickel substitution. This paper analyses the corrosion behaviour and cations released for five austenitic steels, nominally "nickel-free". The analysis of electrochemical parameters, open circuit potential, polarisation resistance, Tafel slopes, corrosion current, breakdown potential, potentiodynamic polarisation curves, and coulometric analysis by zone, reveal that the new austenitic steels, nominally "nickel-free", do not behave in the same way. In the family of steels studied, quite a large dispersion is noted in the corrosion behaviour. With regard to the crevice corrosion behaviour, the steel grades studied can be classified into three groups, with crevice potentials of 600-650mV; 350-450mV and 100-150mV. The release of 18 cations (Al, Ba, Be, Cd, Co, Cr, Cu, Fe, Li, Mn, Mo, Ni, P, Pb, Sn, Sr, Ti, and V) was studied by extraction tests in artificial sweat and bone plasma fluid. The extraction tests reveal that the "nickel-free" steels indeed release only faint traces of nickel. Yet many other elements, some of them potentially harmful, are released in significant amounts. Generally, the amount of cations released is substantially higher in the artificial sweat solution than in the bone plasma.

Evaluation of corrosion on plasma sprayed and anodized titanium implants, both with and without bone cement.

The corrosion behavior of titanium with vacuum plasma sprayed titanium coatings and with anodized surfaces, both with and without polymeric bone cement were evaluated. Electrochemical extraction tests were carried out with subsequent analysis of the electrolyte by ICP-MS in order to verify our hypothesis of the ionic permeability of the polymer cement. The complexity of the situation resides in the existence of two interfaces: electrolyte-polymer and polymer-metal. The surface preparation (treatment of the surface) plays an important role in the corrosion resistance of titanium. The electrochemical magnitudes that were examined reveal that the plasma spray surfaces have the lowest corrosion resistance. The cement, in spite of having reduced electrical conductivity in comparison to metal, is an ionic transporter, and therefore capable of participating in the corrosion process. In the present study, we observed in fact crevice corrosion at the metal-cement interface. In the case of plasma spray surfaces, a process of diffusion of titanium particles in the electrolyte could accompany the crevice corrosion. In this study, we have shown that there is a corrosion process at the surface of the titanium through the cement which has as a consequence on the one hand the formation of titanium cations and on the other hand the growth of a passive layer on the titanium. In conclusion, we identified two principal factors that influence the corrosion process: [1] the type of surface treatment for the titanium, and [2] the ionic conductivity of the cement. There is indeed ionic transport through the cement; as evidenced by the presence of titanium in the electrolyte solution (ICP-MS analysis) and chloride at the surface of the titanium sample (EDX analysis). We show that the polymer cement is an ionic conductor and participates in the corrosion of the embedded titanium. We cannot deduce from our results, however, whether the polymer itself possesses corrosive properties. Long-term experiments will be necessary to study the degradation behavior of the polymer cement.

Pitting, crevice and galvanic corrosion of REX stainless-steel/CoCr orthopedic implant material.

The corrosion behavior of surgical implant CoCr alloy and REX 734 steel has been investigated. The pitting or crevice corrosion potentials have been determined to reach values as high as 500 mV vs. SCE for CoCr and 450 mV vs. SCE for REX 734. The galvanic corrosion behavior of CoCr/REX 734 couples has been evaluated with various electrochemical techniques. The measurement of the corrosion current of the galvanic couple as well as its prediction by applying mixed potential theories on measured potentiodynamic polarization curves revealed low galvanic currents in the range of nanoamperes.

Circadian variations of urinary electrolyte concentrations in preterm and term infants.

OBJECTIVES: To determine whether a circadian variation of urinary excretion of calcium and phosphorus exists in preterm infants. STUDY DESIGN: We studied 70 newborn infants (median birth weight 1920 gm, range 660 to 3550 gm; median gestational age 34 weeks, range 25 to 42 weeks) at a median postmenstrual age of 36 weeks (range 32 to 42 weeks). Within a period of 24 hours, four urine specimens were collected during 6-hour periods. The concentrations of calcium, phosphorus, sodium, potassium, and creatinine were determined and creatinine quotients were calculated for each specimen. RESULTS: No clinically relevant circadian variation in urinary excretion for any of these minerals was found. CONCLUSION: If spot urine specimens are used to monitor calcium and phosphorus balance in preterm infants, the time of the day these are collected is not important.

Deposition of nitrogen-containing compounds to an extensively managed grassland in central Switzerland.

During four intensive observation periods in 1992 and 1993, dry deposition of nitrogen dioxide (NO(2)) and ammonia (NH(3)), and wet deposition of nitrogen (N) were determined. The measurements were carried out in a small, extensively managed litter meadow surrounded by intensively managed agricultural land. Dry deposition of NH(3) was estimated by the gradient method, whereas eddy correlation was used for NO(2). Rates of dry deposition of total nitrate (= nitric acid (HNO(3)) + nitrate (NO(3)(-))), total nitrite (= nitrous acid (HONO) + nitrite (NO(2)(-))) and aerosol-bound ammonium (NH(4)(+)) were estimated using deposition velocities from the literature and measured concentrations. Both wet N deposition and the vertical NH(3) gradient were measured on a weekly basis during one year. Dry deposition was between 15 and 25 kg N ha(-1) y(-1), and net wet deposition was about 9.0 kg N ha(-1) y(-1). Daily average NO(2) deposition velocity varied from 0.11 to 0.24 cm s(-1). Deposition velocity of NH(3), was between 0.13 and 1.4 cm s(-1), and a compensation point between 3 and 6 ppbV NH(3) (ppb = 10(-9)) was found. Between 60 and 70% of dry deposition originated from NH(3) emitted by farms in the neighbourhood. It is concluded that total N deposition is exceeding the critical load for litter meadows, is highly correlated to local NH(3) emissions, and that NH(3) is of utmost importance with respect to possible strategies to reduce N deposition in rural regions.