A percussive drilling component includes a cylindrical male screw thread formed of a steel material. The thread includes thread crests and thread roots interconnected by thread flanks. To protect the thread against corrosion, the thread is coated with a material having a lower electrode potential than the steel material. The coating is situated at least in regions located radially inwardly of the thread flanks. The male screw thread can be attached to a female screw thread of another percussive drilling component, the female screw thread also being coated with the material.

RELATED INVENTION  
         [0001]    This application claims priority under 35 U.S.C. §§ 119 and/or 365 to Patent Serial No. 0000702-1 filed in Sweden on Mar. 2, 2000, the entire content of which is incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a drill element for rock drilling, and to a thread joint for interconnecting the drill element to other drill elements, wherein the thread joint is protected against corrosion. The invention also pertains to a method of protecting a threaded end of a drill element from corrosion.  
         PRIOR ART  
         [0003]    During percussive rock drilling, the drill elements, i.e. bits, rods, tubes, sleeves and shanks adapters, are subjected to corrosive attacks. This applies in particular to underground drilling where water is used as a flushing medium and where the environment is humid. The corrosive attacks are particularly serious in the most stressed parts, i.e., thread bottoms and thread clearances. In combination with pulsating stress, caused by shock waves and bending loads, so-called corrosion fatigue arises. This is a common cause for failure of the drill element.  
           [0004]    Today low-alloyed, case hardened steels are normally used in the drill element. The reason for this is that abrasion and wear of the thread parts have generally limited the life of the drill element. As the drill machines and the drill elements have become more efficient, problems due to abrasion and wear have diminished, and corrosion fatigue has become a major factor in limiting the life of the drill element.  
           [0005]    The case hardening produces compressive stresses in the surface, which gives certain beneficial effects against the mechanical part of the fatigue. The resistance to corrosion in a low-alloyed steel is however poor and for that reason corrosion fatigue still happens easily.  
           [0006]    In U.S. Pat. No. 4,872,515 or 5,064,004 a drill element is shown wherein a threaded portion is covered with a metallic material, which is softer than the steel of the drill element. Thus, it is intended to solve the problem of pitting in the threads by covering at least the parts of the thread of the drill element that cooperate with other parts of the threaded connection.  
         OBJECTS OF THE INVENTION  
         [0007]    One object of the present invention is to substantially improve the resistance against corrosion fatigue of a drill element for percussive rock drilling.  
           [0008]    Another object of the present invention is to substantially improve the resistance against corrosion fatigue in sections of reduced cross-sections in a drill element for percussive rock drilling.  
           [0009]    Still another object of the present invention is to substantially improve the resistance against corrosion fatigue in the roots of the thread in a threaded portion in a drill element for percussive rock drilling.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention relates to a percussive drilling component which has a male screw thread, as well as to the combination of that drilling component attached to another drilling component which has a female screw thread.  
           [0011]    The percussive drilling component is formed of a steel material and includes an integral substantially cylindrical male screw thread. The thread comprises thread crests and thread roots interconnected by thread flanks. The thread is coated with a material having a lower electrode potential than the steel material. The coating on the thread is situated at least in regions located radially inwardly of the thread flanks.  
           [0012]    In the case of the combination wherein the above-described drilling component is attached to another drilling component having a female screw thread, the female screw thread could also be coated with the lower potential material, with the coating situated in regions located radially outwardly of the female thread flanks.  
           [0013]    The invention also pertains to a method of protecting a threaded end of a steel percussive drilling component against corrosion, by coating the entire threaded end with a coating material having a lower electrode potential than the steel material of the drilling component. The coating will thus be disposed on impact regions of the thread where the coating will be worn off during percussive drilling. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The objects and advantages of the invention will become apparent from the following detailed description of preferred embodiments thereof in connection with the accompanying drawings, in which like numerals designate like elements, and in which:  
         [0015]    [0015]FIG. 1 shows a drill element according to the present invention in a side view, partly in cross-section;  
         [0016]    [0016]FIG. 2 shows one end of the drill element of FIG. 1 in a side view;  
         [0017]    [0017]FIG. 3 shows an axial cross-section of a fragment of the end shown in FIG. 2;  
         [0018]    [0018]FIG. 4 shows an axial cross-section of a first embodiment of a thread joint according to the present invention;  
         [0019]    [0019]FIG. 5 shows an axial cross-section of a second embodiment of a thread joint according to the present invention; and  
         [0020]    [0020]FIG. 6 shows an axial cross-section of an alternative embodiment of a drill element according to the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]    A drill element or first drill string component  10  for percussive drilling shown in FIGS.  1  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.  
         [0022]    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 FIG. 4.  
         [0023]    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 FIG. 4 the thread roots  20  of the male thread  14  are provided substantially distant from the associated crests  22  of the female thread.  
         [0024]    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  (FIG. 3), 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.  
         [0025]    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.  
         [0026]    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.  
         [0027]    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.  
         [0028]    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.  
       EXAMPLE  
       [0029]    During so-called production drilling of long holes a drill tube  10  of about 4 m long is used, FIG. 1, 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  (FIG. 2). Flushing water and pulsating tensile stresses lead to corrosion fatigue (galvanic corrosion) that frequently results in fracture.  
         [0030]    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                      
 
         [0031]    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.  
         [0032]    In an alternative embodiment of a thread joint according to the present invention shown in FIG. 5 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, FIG. 5. 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.  
         [0033]    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 FIG. 6, 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 FIG. 6, both of the transitions  30 , 32  could be provided with layers L.  
         [0034]    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.  
         [0035]    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.