Must one wash his hands after scratching a bald area of his head?
No.[1]
[1] If the bald area is not normally covered by his Yarmulka, [such as one with a receding hairline], then the Poskim clearly rule that it does not require any washing at all, and does not prevent one from learning Torah. [Michaber 4:21; Mamar Mordechai 4:11; Kaf Hachaim 4:98; Sheivet Halevy 8:2; Piskeiy Teshuvos 4:24] Furthermore, even if the bald area is normally covered by his Yarmulka, and seemingly requires washing of his hands due to touching the normally covered areas, nonetheless, the Poskim imply that it never requires washing even in such a case. [See Admur 162:2 and 164:2 in parentheses; Elya Raba 162:17; Chayeh Adam 40:19; Levushei Mordechai 1:12; M”B 162:58; Yifei Laleiv 23; Kaf Hachaim 4:74; Piskeiy Teshuvos ibid]
The reason: As Admur rules that it is specifically the scratching between the hairs that causes the evil spirit to rest and prevents one from Torah learning due to the sweat, and hence if he has no hair then there is no reason to require the washing unless it is considered a normally covered part of the body.