Critical role of slags in pitting corrosion of additively manufactured stainless steel in simulated seawater

Abstract Pitting corrosion in seawater is one of the most difficult forms of corrosion to identify and control. A workhorse material for marine applications, 316L stainless steel (316L SS) is known to balance resistance to pitting with good mechanical properties. The advent of additive manufacturing (AM), particularly laser powder bed fusion (LPBF), has prompted numerous microstructural and mechanical investigations of LPBF 316L SS; however, the origins of pitting corrosion on as-built surfaces is unknown, despite their utmost importance for certification of LPBF 316L SS prior to fielding. Here, we show that Mn-rich silicate slags are responsible for pitting of the as-built LPBF material in sodium chloride due to their introduction of deleterious defects such as cracks or surface oxide heterogeneities. In addition, we explain how slags are formed in the liquid metal and deposited at the as-built surfaces using high-fidelity melt pool simulations. Our work uncovers how LPBF changes surface oxides due to rapid solidification and high-temperature oxidation, leading to fundamentally different pitting corrosion mechanisms

The impact of air power on navies

This thesis examines how air power has affected navies using the case of the United Kingdom between 1945 and 1957. Air power has given rise to numerous theories about its effect on the use of force, in which its impact on navies has been a particular theme. Many thinkers have interpreted air power as a strategic, operational and tactical challenge to navies, which would render them redundant and obsolete. Such ideas originated between the two World Wars but have continually reappeared since 1945 and were often influential in the British debate. During the period under consideration, the Royal Navy was challenged in a series of defence reviews. Although these serious and repeated attacks were generally motivated by financial considerations, they were justified primarily by claims relating to air power. It was argued that nuclear-armed air power would be sufficient to win wars, while any nuclear war would leave little role for navies. Later, this argument shifted to an emphasis on deterrence of war and it was suggested that naval forces were not vital to this deterrent. Although the Admiralty continued to justify a capability to defend sea communications as both a deterrent to war and vital in it, this case did not find favour with the government. During the mid-1950s, however, an alternative focus for the Navy emerged in the form of policing and limited wars, east of Suez. Conflicts in Korea and the Suez demonstrated that such conflicts could occur and that mobile naval forces, particularly carrier aviation, were well suited to intervention in them. It was also argued that land-based aircraft could take over some of the roles traditionally performed by naval forces, notably that of strike (against warships and shore targets) but also that of protecting shipping. These arguments were successfully defeated by the Admiralty. Land-based aircraft proved a useful complement to warships and naval aviation in protecting shipping but could only replace them in very limited areas. The Royal Navy differed from the US Navy in its conception of power projection against the land. Its capability for this task was thrown increasingly into doubt by growing Soviet submarine and air power and came to be justified by east of Suez commitments.</p