a drill element or first drill string component 10 for percussive drilling shown in fig1 to 4 is in the form of a drill tube provided at one end with a sleeve or female portion 11 having a cylindrical female ( internal ) screw thread 12 . the female portion 11 constitutes an integral part of the drill tube 10 . at its other end the drill tube 10 is formed with a spigot or male portion 13 provided with a cylindrical male screw thread or cylindrical external screw thread 14 . the shown thread is a so - called trapezoid thread but other thread shapes can be used , for example a rope thread . furthermore , the drill element has a through - going central flush channel 15 , through which a flush medium , usually air or water , is transferred . in use , a plurality of the components 10 are screwed together , i . e ., the male portion 13 of one component 10 is screwed into the female portion 11 of another , identical component 10 , as depicted in fig4 . the male thread 14 comprises thread flanks 16 , 17 and thread roots 20 arranged between the flanks . the female thread 12 comprises the thread flanks 18 , 19 and thread roots 21 arranged between flanks . in a tightened joint shown in fig4 the thread roots 20 of the male thread 14 are provided substantially distant from the associated crests 22 of the female thread . according to the present invention , regions of reduced cross - section of the male portion , e . g ., the thread roots 20 , restrictions 24 , and clearances , are provided with a coating formed of at least one surface - modifying , corrosion - resistant layer l . the greatest layer thickness is 0 . 002 - 5 mm , preferably 0 . 02 - 2 mm . the thread root has a first width , w 1 ( measured in a direction parallel to the axis of the component 10 ). the thread , that is the thread crest 23 and the uncoated part of the thread flanks 16 , 17 have a second width , w 2 ( fig3 ), wherein the ratio w 1 / w 2 is 0 . 02 - 1 . 2 , preferably 0 . 3 - 0 . 8 . for example , a rope thread ( of designation r 35 ) was covered by a 5 mm thick coating ( w 1 ). the thread pitch was 12 . 7 mm , resulting in w 2 being 7 . 7 mm ( i . e ., 12 . 7 minus 5 ). also , w 1 / w 2 = 0 . 65 . said corrosion - resistant layer l in the coating of the drill element according to the invention is less noble than the carrying or underlying steel of the component 10 . that is , the layer has a more negative electrode potential by at least 50 mv , preferably by at least 100 mv in the actual environment . that difference in electrode potential then functions as a cathode protection where the coating constitutes a galvanic anode ( sacrificial anode ). examples of such protective materials are aluminum , zinc and magnesium as well as alloys of these , preferably zinc alloys . the remaining layers can be constituted of binder layers in order to increase the bond between the coating and the steel . a number of different coating methods can be used to apply the layer , for example hot dipping , chemical or electrolytic plating or thermal spraying . in case the coating process produces a coating which cover more than some of the sections of reduced cross - section , e . g ., the entire thread , the excess coating portions can be machined off before the tube is used . alternatively , the excess coating portions could be allowed to wear of during use . in that regard , it will be appreciated that after the steel tube 10 has been screwed together with the female thread of another tube during the formation of a string , parts of the two threads will be in contact with one another . during a percussive drilling operation , the coating at those contact or impact regions , i . e ., the excess coating portions which are not needed , will be quickly worn - off , leaving the coating intact at the regions where corrosion protection is especially needed , i . e ., at the regions of reduced cross section that are exposed to corrosive attacks . a number of different coating methods can be used to apply the layer l , for example hot dipping , chemical or electrolytic plating , or thermal spraying . tube 10 has been screwed together with the female thread of another tube during the formation of a string , parts of the two threads will be in contact with one another . during a percussive drilling operation , the coating at those contact or impact regions , i . e ., the excess coating portions which are not needed , will be quickly worn off , leaving the coating intact at the regions where corrosion protection is especially needed , i . e ., at the regions of reduced cross section that are exposed to corrosive attacks . thus , it is possible within the scope of the invention to coat most or all of the drilling component , whereafter the coating portions disposed at regions where the drilling component contacts an adjacent drilling component will wear away quickly . during so - called production drilling of long holes a drill tube 10 of about 4 m long is used , fig1 which is combined with others to form a long string , i . e ., eight tubes of case - hardened low - alloy steel were employed in the string . the critical parts of the tubes from a corrosion standpoint are the bottoms ( roots ) 20 of the external threads 14 ( fig2 ). flushing water and pulsating tensile stresses lead to corrosion fatigue ( galvanic corrosion ) that frequently results in fracture . the eight tubes of case hardened , low - alloyed steel were coated with a layer of zinc with a thickness of about 0 . 2 mm by dipping in a bath of molten , zinc , so - called dip galvanizing . zinc has an electrode potential of about − 860 mv in seawater at 20 ° c ., which shall be compared to − 500 mv for low - alloyed steel . the zinc layer was machined from the thread flanks by means of a rotating brush . then drilling was performed in a rig for drifter drilling underground until fracture or the tubes were worn - out . following life spans for the eight tubes , measured in drilled meter , were obtained : test no . drilled meters 1 4297 2 2489 3 3210 4 2041 5 3933 6 4268 7 3085 8 2608 normal life spans for uncoated drifter tubes of conventional type steel are about 2000 m at the actual test place where the rock substantially consists of granite , which shows that the use of a drill steel coated according to the invention gives a striking improvement . in an alternative embodiment of a thread joint according to the present invention shown in fig5 also the thread 12 ′ of the female portion 11 ′ would be coated with a layer of a material of lower electrode potential than the low - alloy steel , fig5 . in other words , sections of the female portion 11 ′ of reduced cross - section would be provided with a coating constituting a sacrificial anode . only the most exposed portions , that is , sections of reduced cross - section such as thread roots 21 ′, restrictions and clearances would preferably be coated . everything stated above about the coating l , including all of the thickness and width characteristics , applies also to the case where the coating is applied to the female portion 11 ′. for example , the entire female thread could be dipped in a bath of coating material , whereupon the coating at the impact regions would wear away during drilling . in another alternative embodiment of a drill element according to the present invention only the most stressed parts of the thread root would be coated . for example , as shown in the right half of fig6 only one of the two transitions 30 , 32 between the thread root 20 and the flank of a trapezoidal thread would be provided with a layer l . alternatively , as shown in the left half of fig6 both of the transitions 30 , 32 could be provided with layers l . the invention consequently relates to a thread joint and a drill element for percussive drilling with a restricted portion which is coated by a corrosion - resistant layer in order to substantially improve the resistance to corrosion fatigue . the layer is preferably discontinuous in the axial direction of the tube to avoid deposition on and softening of the thread flanks . although the present invention has been described in connection with preferred embodiments thereof , it will be appreciated by those skilled in the art that additions , deletions , modifications , and substitutions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims .