The file history of the US “Titanium” patent (US 2,205,854)
The US patent application claimed priority from two German patent applications, filed respectively on 10 July 1937 (K147,211) and on 7 October 1937 (K148,168). Upon grant, however, only the earlier one of the applications was retained by the US Patent Office as priority claim.
This is rather surprising since the July ’37 application related to the use of calcium in the production method and the October ’37 application related to the use of magnesium which is precisely what was claimed in the US patent.
The explanation for the renouncement to the later priority claim is to be found in the fact that Kroll’s priority date had to predate a conflicting US Patent granted to Freudenberg  which carried an effective US filing date of 1 September 1937. By selecting the first application only as priority application Kroll avoided having to deal with Freudenberg’s “prior art” patent.
The US Patent Office, however, allowed a generic claim for the use of alkaline earth metals, as well as a specific claim to the use of magnesium.
The 10 July 1937 application issued in Germany as a patent (DE patent 674,625) on 30 March 1939. Upon filing the application Kroll opted for an extension of the patent protection to Austria.
The fate of the 7 October 1937 application remains unknown.
Maybe, in order to safeguard his US patent, Kroll also renounced pursuing the second application in Germany to remain consistent with the position taken in the US patent application procedure. Unlike the US patent, the German patent, however, did not contain a generic claim for the use of alkaline earth metals but was limited to the use of calcium. 
Two authors suggest that in 1939 Kroll conducted experiments with magnesium as reducing agent in Germany, pointing towards the German companies DEGUSSA and OSRAM. 
The complete text of the German applications has been preserved as “priority” documents in the corresponding US patent prosecution file which is kept in the National Archives in Kansas City, Missouri, USA.
From the US patent specification, it is clear that Kroll discovered between July and October 1937 that the use of magnesium was preferable to the use of calcium in terms of the costs of the materials and in the working temperature range. (The date of 30 July 1937 is reported as the date on which Kroll achieved the major breakthrough in his quest for producing ductile titanium.) 
N.B. : Patents do reflect the course of history as can be seen by the fact that in Kroll’s German patent the original Swastika in the Reichsadler was blacked out after WW2 in the patent collection of the German Patent Office; however, the notification of the fact that the “Third Reich” patent was valid in the annexed Austrian territory, as of its date of grant, was not barred.
The specification of the patent filed by Kroll in the USA on 6 July 1938 was much more elaborate than the specifications of the priority applications filed in Germany a year earlier. During that year Kroll probably collected more data in support of his titanium producing process which he then included in the US application.
The US patent examiner was quite critical during the examination procedure and requested that Kroll submit a drawing of the furnace described in his application:
“since the disclosure is based so closely upon the operation of the furnace, a drawing of the furnace described on pages 11 and 12 is required”
The following drawings were subsequently presented to the Examiner:
Kroll’s agent, when asked to reduce the number of claims, i.e. reduce the scope of protection granted by the patent, stated the following:
“Applicant is of the impression that the significance of the present invention is greater than the Examiner may have assumed. The fact that applicant's method allows producing titanium in relatively large quantities and on a satisfactory economic basis, which titanium is ductile and even cold malleable, is of such importance over the many attempts heretofore made, that the number of claims should be adequate to satisfactorily cover the invention.
The patent issued in the name of Wilhelm Kroll on 25 June 1940.
 US Patent 2,148,345
 It has to be to pointed out here that Kroll’s titanium process was only patented in Germany (including Austria), in the United Kingdom and in the US; since Kroll’s German patent was limited to the use of calcium, magnesium could be used as reducing agent in Germany without Kroll’s consent, i.e. without a license under Kroll’s German patent.(see footnote 38).
 Titanium - A Materials Survey 1957, Jesse A. Miller, United States Department of the Interior, September 1957, page 105: “Kroll’s experimentation with the magnesium reduction process in Germany until 1939 resulted in its adoption by Degussa and Osram, two large German manufacturing firms. Degussa produced titanium in its Frankfurt am Main plant from 1938 to 1942, reaching a maximum yearly output of 1,050 pounds in 1941. The Osram plant turned out approximately 65 pounds per month from 1942 to 1944. Osram has not reported any production since World War II, but Degussa was again operating a pilot plant at Frankfurt am Main in 1955. Titangesellschaft, a subsidiary of the National Lead Co., had a pilot plant at Leverkusen, Germany, which had a capacity of 2.2 short tons per month in 1954.”
 The titanium industry: a case study in oligopoly and public policy, dissertation, Francis George MASSON, The Ohio State University, 1954, page 42
 Black Sand : “The history of titanium”, Kathleen L. Housley, 2007, page 14