Venus famously has clouds of concentrated sulfuric acid in its upper atmosphere. Frequently I will see posts here citing the acidity of the atmosphere as a challenge to probes on the surface (such as "What material properties would be necessary to shield a lander from the environment of the Venusian surface?"). But would there be appreciable quantities of acid at the surface? Is sulfuric acid stable at those temperatures?
We don't need to invoke sulfuric acid or sulfur oxides. Even at relatively low partial pressures and temperatures close to those found on the surface of Venus, carbon dioxide alone can oxidize iron. Thus we need a metal more robust than common steel to avoid being corroded on Venus. See for example Ref. 1, which studies the impact of carbon dioxide on iron catakysts.
1. Ewa Ekiert and Walerian Arabczyk, "Passivation versus Oxidation of Iron Catalyst with Carbon Dioxide", J. Phys. Chem. C 2015, 119, 8, 4000–4008
Recently, I answered this question. I came to know that descent probes also provided some evidence for thin aerosol layers near the surface. It is written that:
A recent reanalysis of Venera-13, -14 descent probe spectrophotometer data found a sharp decrease of light levels at 1–2 km altitude, interpreted as indicating a detached layer of aerosols of unknown nature at this level. The authors point out that its altitude is similar to that at which radar-bright deposits attributed to metallic condensate on the mountain tops have been found . The aerosol layer could also be associated with volcanic ash or dust lifted by wind or near surface sulfuric acid haze. Further investigation of such low-altitude hazes may be possible by radar investigations or near-infrared spectroscopy on the nightside.
So there might be a minute amount of sulfuric acid just above the surface but that should not be our concern. If we want to protect our spaceship or lander from being corroding, we should concentrate on the vast amount of sulfuric amount present on the clouds.
- Titov, D.V., Ignatiev, N.I., McGouldrick, K. et al. Clouds and Hazes of Venus. Space Sci Rev 214, 126 (2018). DOI: 10.1007/s11214-018-0552-z