https://1997-2001.state.gov/www/regions ... turkey.pdf
The basic story is, Germany was Turkey's biggest trading partner in the 1930s, and Turkey was Germany's principal supplier of chromium. But Turkey got spooked when Italy invaded Albania and concluded the Pact of Steel with Germany in 1939, so Turkey tried to align itself with Britain. In 1939, Turkey concluded a trade deal with Britain wherein Turkey agreed to sell all of its chromium to Britain through 1941. So Germany had to look elsewhere for chromium, mainly the Soviet Union. After Barbarossa started, Germany had to rely on its conquests in the Balkans and Norway for chromium.
In October 1941, Turkey agreed to resume chromium shipments to Germany in 1943, in the amount of 45,000 tons, which the Allies estimated to be Germany's annual requirement. Turkey agreed to sell even more chromium to Germany in 1944, but the Allies finally put their foot down and Turkey let its trade agreement with Germany expire in April 1944.
There is an interesting quote from Speer in the article:
The article also notes that manganese and chromium can be substituted for some purposes, so presumably some of Germany's chromium needs were alleviated by their exploitation of the manganese mines in the Ukraine after Barbarossa.Albert Speer, who reported on the current German inventory of alloy metals in amemorandum to Hitler on November 10, 1943, and concluded:
"Hence the element in shortest supply is chromium. This is especially grave since chromium is indispensable to a highly developed armaments industry. Should supplies from Turkey be cut off, the stockpile of chromium is sufficient only for 5.6 months. The manufacture of planes, tanks, motor vehicles, tank shells, U-boats, and almost the entire gamut of artillery would have to cease from one to three months after this deadline, since by then the reserves in the distributions channels would be used up."
In his memoirs, Speer explained further that the conclusion in his memorandum "meant no moreor less than that the war would be over approximately ten months after the loss of the Balkans."