Iron-based biodegradable implants need a corrosive behavior which is more rapid compared to pure Fe and still controlled enough not to induce oxygen deficiency or chemical instability of the surrounding tissue interface. Near-surface pH and dissolved oxygen values have been studied to see whether the near-neutral pH may hide oxygen starvation in powder-processed Fe, Fe35Mn, and \femnag\ in Hanks' Balanced Salt Solution (HBSS) both without and with addition of calcium salts. The near-surface pH and dissolved oxygen were measured \SI{50}{\micro\meter} away from alloy surface under flowing conditions at \SI{37}{\celsius}. The results have been interpreted in connection with the powder-processing technology, composition of the studied alloy and chemistry of the Fe--Mn--Ag system in the electrolyte. pH range, normalized oxygen deficit and oxygen-pH discrepancy ratio are used as criteria of visibility of the interfacial corrosion process. In the calcium-free HBSS \femnag\ revealed the most pronounced micro-galvanic effects with pH variation of 6.40 to 8.10 and dissolved oxygen drop below \SI{0.50}{mg.L^{-1}}. This case was obviously aggressive as oxygen deficit occurred together with wide pH variations. Different hazard was found in calcium-containing HBSS where early Ca-P precipitates inhibited the oxygen uptake above \femnag\ but after 24 hours the oxygen minimum dropped to \SI{0.40}{mg.L^{-1}} whereas pH varied in the range of 7.16 to 7.31. Oxygen-pH discrepancy ratio thus became high indicating that Ca-P precipitation may hide pronounced cathodic oxygen uptake. Fe35Mn alloy demonstrated moderate pH separation but no oxygen uptake inhibition, while pure Fe was sensitive to product-covered oxygen deficiency. The results prove that Fe-Mn-Ag biodegradable alloys cannot be characterized only by corrosion acceleration or pH moderation.

biodegradable iron,Fe35Mn,Fe--Mn--Ag,local pH,dissolved oxygen,Hanks' Balanced Salt Solution,calcium phosphate,powder metallurgy