Patent Publication Number: US-3875027-A

Title: Method of electroplating tubing prior to terne alloy coating

Description:
United States Patent 1191 Gondek Apr. 1, 1975 METHOD OF ELECTROPLATING TUBING 2.371.725 3/1945 Young 204/38 5 To TERNE ALLOY COATING FOREIGN PATENTS OR APPLICATIONS Inventor: Stanley Gondek, y. Mich- 454.415 9/1936 United Kingdom 204/38 s [73] Assignee: Bundy Corporation, Detroit. Mich.  
  Primary ExamineF-R. L. Andrews [22] Flled: June 1973 Attorney, Agent, or Firm-Harness. Dickey &amp; Pierce [21] Appl. No.: 374,883  
  [57] ABSTRACT 52 U.S. Cl. 204/28, 204/38 s, 204/40, Ferrous metal is Provided with corrosion resistant 204 29 204 49 204 52 y coating having a surprisingly great ability to retard the 5 1 CL n 323 5 5 23 5 50 23 17/00 formation of rust when subjected to salt spray testing. 5 Fie|d f Search U 20 33 5 25 23 29 40 Successive thin layers of copper and nickel are elec- 204/49, 52 Y troplated on steel tubing after which the tubing is terne coated by hot immersion. The corrosion resis- 5 References m lance which is achieved by this combination of layers UNITED STATES PATENTS greatly exceeds the sum of the corrosion resistance of 626,994 6/1899 Francis 204/38 5 each layer when used alone 2,268,617 H1942 Pierce 204/38 S 6 Claims, 6 Drawing Figures SHEET 2 Bf 2 METHOD OF ELECTROILATING TUBING PRIOR TO TERNE ALLOY COATING SUMMARY OF THE INVENTION Steel tubing is used on automobiles in various locations where it is exposed to road salt and other corrosive elements. For example, copper brazed steel tubing of the general type shown in Bundy US. Pat. No. 1,431,368 has been used for automobile hydraulic brake lines for approximately 40 years. Such copper brazed steel tubing and other forms of steel tubing are also used for automobile fuel lines. Both brake and fuel lines extend through exposed locations on the underbody of the car where they come into contact with water, road salt and other elements which accelerate their corrosion. It has been customary to apply a terne coating of tin-lead alloy to steel tubing used on automobiles to inhibit the corrosion of the tubing. The terne coating is customarily applied by passage of the tubing through a bath oftin-lead alloy which is maintained at a temperature of 700-750 F. Various improvements in terne coating techniques have been developed which have increased the thickness of terne coating which can be applied to the tubing. Nevertheless, terne coated steel tubing will begin to rust after about 24-72 hours of standard salt spray testing. The inception of visible rust may be regarded as the beginning of the gradual structural breakdown of the tube wall. Modern automotive safety requirements have created a demand for tubing capable of longer exposure to a corrosive environment before showing rust.  
  Various techniques for improving the corrosion resistance of steel tubing have been proposed. For example, copper brazed tubing has been made from a special laminate in which a layer of stainless steel is sandwiched between two layers of low carbon steel. However, such special laminated materials are extremely expensive. Furthermore, the outer low carbon steel layer will still exhibit early signs of rust in the same manner as steel tubing made in the conventional manner. Such tubing is, therefore, not subject to visible inspection to estimate its soundness. Visual inspection for rust is the most convenient and usable method of inspecting steel tubing once it has been placed in use.  
  Still other proposals have been made for coating steel tubing with materials which are well known for their corrosion resistance. For example, steel tubing electroplated with thin layers of copper and nickel up to .001 inch in total thickness gave erratic and unpredictable results when the tubing was subjected to salt spray testing. Very few of a number of tested samples survived 72 hours of salt spray testing without showing rust. While increasing the thickness of a copper or a nickel coating on a steel tubing will increase the corrosion resistance roughly in proportion to thickness of the applied coating, electroplated metal coatings are quite expensive, the amount of the cost varying almost in direct proportion to the thickness of the coating.  
  Tests have been conducted on tubing made from steel clad with monel and other corrosion resistant alloys. Such tests have generally not been successful. The presence of a nickel alloy on the outer surface of the steel makes it impossible or difficult to furnace braze such tubing. Electrical resistance brazing, on the other hand, is slow. costly, and has generated undesired contaminates on the interior of tubing made from monel clad steel. Butt welding tubing from such materials destroys the cladding at the seams, and exposes the steel substrate to corrosion. Most importantly, however, such clad materials are extremely expensive and tubing made from them has not been sufficiently competitive in price to be of great interest.  
  The replacement of steel tubing with tubing made from non-ferrous materials has a number of drawbacks. Steel tubing possesses great structural strength, is low in cost, has a proven high reliability, and a substantial body of knowledge exists with respect to the fabrication of steel tubing into various shapes. Copper, on the other hand, is more expensive, is lower in strength and is frequently in short supply. With respect to tubing intended for automobile use, copper has the further detriment of impairing the scrap value of a car. Nickel and nickel-rich alloys are so much more expensive than steel as to be virtually noncomparable to steel tubing. It will, therefore, be apparent that a low cost means of providing a substantial improvement in the ability of steel tubing to withstand corrosion is of substantial importance, particularly to the automotive industry.  
  The present invention is characterized by the discovery that a terne coating of conventional composition applied in the molten condition over a relatively thin primary coating of copper and/or nickel electrodeposited on ferrous substrate will give the substrate a surprisingly high degree of corrosion resistance. Sample lengths of copper brazed steel tubing which were first given a 0.0005-0.00l5 inch total coating comprising electroplated layers of both copper and nickel and thereafter passed through a hot terne alloy bath were found to withstand 3,000-4,000 hours of salt spray testing before exhibiting rust. The surprisingly high degree of corrosion resistance of such a thin and economically practical combination of coatings was completely unexpected. The terne coating may be applied over a primary coating consisting of copper or nickel alone, but the best results were obtained when using a primary coating consisting of a copper layer followed by a layer of nickel.  
  The corrosion resistant coating of the present invention has been developed in connection with steel tubing. However, the coating and method of this invention is equally applicable to other forms of ferrous substrates.  
 DESCRIPTION OF THE VIEWS OF THE DRAWINGS FIG. 1 is a cross sectional view of typical tubing to which the corrosion resistant coating of the present invention is applied;  
  FIG. 2 is an enlarged exploded sectional view of the structure shown in FIG. 1, taken along the line 22 thereof, showing the coating layers which are applied according to one embodiment of the present invention;  
  FIG. 3 is a schematic view showing one preferred processing of tubing made in accordance with the present invention; and  
  FIGS. 4, 5 and 6 are diagrams showing different combinations of layers applied to a ferrous substrate in accordance with the present invention.  
 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 illustrates the cross sectional shape ofa typical copper brazed steel tube 10 used for automotive brake lines and to which the corrosion inhibiting coating of the present invention has been applied. The tube 10 is made from a steel strip 12 having a nominal 0.00015 inch thick copper plating on both surfaces. The strip 12 is preferably made from a low carbon steel such as test as well as thicknesses of copper and nickel electroplating on the tested tubes can be found in Table l. One-half of the total electroplated coating of each sam ple was copper with the other half consisting of nickel AISl-l008 steel. The strip 12 is rolled twice around latapplied over the copper.  
 erally into the double walled configuration illustrated in FIG. 1 and is then furnaced brazed to cause the copper plating to form a copper joint 14 between the over- TABLE I lapping portions of the strip 12. Some of the copper Tomi c m Hours i a Electroplating Terne Until which is plated onto the str p 12 prior to rolling nd Tube NU Process Thickness Coated Rust brazing remains on the outer peripheral surface 16 of the strip 12 although the uniformity ofthickness of this 367 1 0.00070&#34; No 48 coating is impaired by the brazing. Furthermore, the 323 l g&#39;gggzgh Z brazing causes some diffusion of copper into the steel. 370 E 000043&#34; No 43 It has heretofore been common practice to pass the 372 3 No 139 tube 10 through a hot terne alloy bath to terne coat the 373 4 000130&#34; N 211 tub o surfa l .The tern metalc n&#39; sofS-ZS 374 4 (100033&#34; N0 72 e Ce 6 e 0 5m 375 5 0.00072&#34; Yes 4,776 percent tin and the balance lead. It IS preferred to use 376 5 000077&#34; No 72 a terne metal having about l57r tin. Typically, the 377 6 1: 72 quantity of the terne coating which is applied to the 2- 3 2 838695 g E tube 10 according to current commercial practice is 380 7 000063;&#34; No 48 about 0. l6 ounce per square foot of the area of the sur- 2 81883 32,. 3% face 16. The thickness of this coating is approximately 383 9 0.00172&#34; No 48 384 9 0.00077&#34; No 48 0.18 mil. Such terne coated copper brazed steel tubing 385 0 No 48 is typically able to withstand about 24 to 72 hours of 3 5 10 0,000 37&#34; N0 72 387 11 0.00042&#34; No 48 standard salt spray testing before exhibiting rust on its 388 H No 72 outer surface. Modern automotive safety requirements 389 0 No 72 have, however, created a need for tubing which will en- 12 000100&#34; NO g 391 13 000030&#34; No 48 dure substantially longer periods of salt spray testing without beginning to rust.  
  In an effort l l a lhhlhg havlhg suhstahhahy While some improvement was noted for a few of the Proved Corrosion reslstahfienumber of speclmeflsPf tubes in the test, no one electroplating procedure was PE brhled Steel tuhlhg were eleFtmPlated found to give superior results on both of the tubes elecvarying thicknesses of copper and nickel. The total troplated by the Same procedure or Outstanding thickness of material electroplated on each specimen results on either f the tubes f which it was used never exceeded 0.002 inches. When such plated tubes However the one tube which was dipped in hot terne l&#39; suhlected to 53h p l testing, y gave results alloy after being electroplated with first copper and WhlCh roughly equivalent to terne coated Steel then nickel gave unbelievably outstanding results. tubing. During one initial test lot none of the tubes sur- 40 The result shown in Table 1 f tube N0 375 (the 72 hours 0f p y teshhg l&#39;ushhg and only tube which was terne coated after being electromost of the tubes showed rust after only 24 hours De plated) was completely unexpccted d Surprising spite such poor results, it was decided to test a series of I i f h resuhs hi d i h b N 375 3 tubes f were first PPP Pl and hlchel second test series was arranged comparing the results plated In accordance with 13 different electroplating f wi or not app|ying, terne coating over a i. f The PTOdures dlfferefl from one ail-Other mary electroplated coating of varying thickness. This pnhclpally the p p &#39;l Qleahmg steps and 1n h test was also designed to evaluate different combinause of strike? or flash plating baths before the main tions f layers ki up h i a y ele tro lated plating baths in some of t proce es T ntycoating. The results of this further test are given in tubes. 8 inches long h ing hr n h Inch Table [I and they confirm the fact that greatly im- Outer diameter and a 002 l thlckHeSS were Used In proved corrosion resistance for a steel substrate can be this test series. One of the tubes was also terne coated Obt ined b the combination of a relatively thin elecafter it was electroplated. All 25 tubes were then subtroplated primary coating followed by a secondary jected to a standard salt spray test. The results of this terne coating applied by the hot dip process.  
 TABLE II Electroplated Electroplated Total Terrie Hours Until Tube No. Copper Nickel Plate Coated Rust 45s 0.00035&#34; 0.00035&#34; 0.00070&#34; No 24 46l 0.00035&#34; 0.00035&#34; 0.00070&#34; Yes 2,210 459 0000375 0.000375&#34; 0.00075 No 24 400 0.000375&#34; 0.000375&#34; 0.00075&#34; Yes 1,970 446 0.000375&#34; 0.000375&#34; 0.00075&#34; No 24 447 0.000375 0.000375&#34; 0.00075&#34; Yes L970 448 0.00050&#34; 0.00050&#34; 0.00l No 96 449 0.00035&#34; 0.00035&#34; 0.00070&#34; Yes L682 454 0.000375&#34; 0.000375&#34; 0.00075&#34; No 24 455 0.000375&#34; 0.000375&#34; 0.00075&#34; Yes 3.143 456 0.00050&#34; 0.00050&#34; 0.001&#34; No 24 457 0.00050&#34; 0.00050&#34; 0001&#34; Yes 3,143 402 0.00075&#34; 0.00075&#34; N0 72 403 0.00015&#34; 0.00015&#34; Yes 552 TABLE ll-Continued Elcctroplated Electroplated Total Tcrne Hours Until Tube No. Copper Nickel Plate Coated Rust 464 0.00075&#34; 0.00075&#34; Yes 1.178  
 467 0.00040&#34; 0.00041 Yes 3.143  
 469 000045&#34; 0.00045&#34; 0.00090&#34; Yes 3,143  
 470 0.00075&#34; 0.00075&#34; 0.00l50&#34; No 24 471 0.00075&#34; 0.00075&#34; 0.00150 Yes 4.775  
 424 0.000275&#34; 0.000275&#34; 0.00055&#34; Yes 3.791  
 425 0.000375&#34; 0.000375&#34; 0.00075&#34; Yes 6,279+  
 453 0.00040&#34; 0.00040&#34; 0.00080&#34; Ycs 1.682  
 45 1 0.00040&#34; 0.00040&#34; 0.00080 Yes 3.143  
 + test continuing no rust visible Still further tests were conducted in which a third spray testing before beginning to rust.  
 metal was electroplated on the tube prior to hot terne application. By way of example. four tubes in this te In still another test series 91 sample tubes of the type were given the following layers of primary electroplfltdescribed in FIG. 1 were electroplated and all were ing: 0.00017 inch copper. 0.00024 inch nickel and passed through a hot terne alloy to determine how thin 0.00013 inch tin. The tubes were then terne coated by a primary electroplated coating could be utilized satisthe hot application process. The thickness of the time factorily to achieve the improvement which charactercoating was approximately 0.00018 inch. These sample izes this invention. This test series is reported in Table tubes endured between 840 and 1,008 hours of salt 11].  
 TABLE II] Electroplated Electroplated Electroplated Total Hours Until Tube No. Copper Nickel Copper Plate Rust 565 0.00017&#34; 0.00017&#34; [44 566 000017&#34; 0.00017&#34; 312 567 0.000l7&#34; 0.00017&#34; 1 44 568 000023 0.00023&#34; 360 569 0.00023&#34; 000023&#34; 552 570 0.00023&#34; 0.00023&#34; 144 57] 0.00023&#34; 0.00023&#34; 360 56| 0.00040 0.00040&#34; 552 562 0.00040&#34; 0.00040&#34; 888 563 0.00040&#34; 000040&#34; 816 5 0.00045&#34; 0.00045&#34; 888 541 0.00045 0.00045&#34; L488 542 0.00045 000045&#34; 672 543 0.00045&#34; 0.00045&#34; 888 604 000075&#34; 0.00075&#34; 1.104 (:07 0.00075&#34; 0.00075&#34; 1.104 605 0.00085&#34; 0.00085&#34; 2,280 606 0.00085&#34; 0.00085&#34; 408 558 0.00022&#34; 0.00022&#34; 144 572 0.00022&#34; 0.00022&#34; 384 575 0.00022&#34; 0.00022&#34; 216 574 0.00022&#34; 0.00022&#34; 24 551 0.00040&#34; 0.00040&#34; 480 544 0.00040&#34; 000040&#34; 480 545 0.00040&#34; 0.00040&#34; 672 550 0.00050&#34; 0.00050&#34; 8|) 546 0.00050&#34; 0.00050 8 l b 547 0.00050&#34; 000050&#34; 672 557 0.00017&#34; 000017&#34; 408 548 0.00017&#34; 000017&#34; 216 549 0.00017&#34; 0.00017&#34; 672 578 50% 0.00033&#34; 144 584 50% 50% 000033&#34; 840 585 50% 50% 0.00033&#34; 888 586 50% 50% 0.00033&#34; 1.104 577 50% 50% 0.00045&#34; 528 580 50% 50% 0.00045&#34; 528 582 50% 50% 0.00045&#34; 1.104 576 50% 50% 0.00053&#34; 888 581 50% 50% 000053&#34; 888 602 50% 50% 0.00053 1.848 583 50% 50% 0.00060&#34; 888 601 50% 50% 0.00060&#34; 888 587 50% 50% 0.00070&#34; 888 600 50% 50% 0.00070 1.488 583 50% 50% 0.00035&#34; 312 593 50% 50% 0.00035&#34; 360 7 8 TABLE III Continued Elcctrnplatcd Elcctroplatctl Electruplatctl Total Hours Until Tube Nu. Copper Nickel Copper Plate Rust 589 50G 50&#39; 0.00045 1,488 596 50); 504&#39; 000045&#34; 888 598 5095 50% 000045&#34; 672 590 50% 50% 000050&#34; 672 592 50% 50% 0.00050&#34; Lost 595 50% 50% 000050&#34; 408 597 509) 50% 000050&#34; 528 591 50% 5055 0.00060&#34; 1 104 599 509i 50% 0.00060&#34; 888 624 50&#39;; 5071 0.00030&#34; 984 625 50 1 50% 0.00030&#34; 216 626 507: 507: 000030&#34; 360 627 509 50% 0.00030&#34; 480 617 50% 50V: 0.00045 1,488 621 50% 506i 0.00045 1,104 62 2 50% 50% 0.00045 1,104 623 50?; 50% 0.00045 888 612 50&#39; 50% 0.00070 1,488 613 50% 505% 0.00070&#34; 2,280 616 507r 50% 0.00070&#34; 672 614 50% 5071 0.00080&#34; 1,848 615 50% 50% 0.00080&#34; 1,488 618 50% 509&#39; 0.00080&#34; 1,488 645 0.00021 0.00021 360 646 0.00021- 0.00021&#34; 48 647 0.00021&#34; 0.00021&#34; 168 653 0.00049&#34; 0.00049&#34; 528 654 0.00049&#34; 0.00049&#34; 672 655 0.00049&#34; 000049&#34; 528 648 0.00061&#34; 0.00061 528 649 0.00061&#34; 000061&#34; 528 629 0.00020&#34; 0.00020&#34; 72 630 0.00020&#34; 0.00020&#34; 72 631 0.00020&#34; 0.00020&#34; 216 632 0.00032&#34; 0.00032&#34; 360 633 0.00032&#34; 0.00032&#34; 672 635 0.00032 000032&#34; 360 636 0.00060&#34; 0.00060&#34; 1.032 638 0.00060&#34; 0.00060&#34; 672 637 000065&#34; 0.00065&#34; 528 639 0.00065 0.00065 1,032 640 0.00075&#34; 0.00075&#34; 528 642 0.00075&#34; 0.00075&#34; 48 643 000075&#34; 0.00075&#34; 5221 While the total hours of salt spray testing without rust reported in Table lll are not as favorable as the terne coated samples of Tables I and II (even in the case of samples plated to the same thickness as the samples of Tables I and 11), consistently high corrosion resistance was experienced when the total electroplated coating thickness was at least 0.0004 inch thick. Only one sample tube having a primary coating greater than this thickness (e.g., tube No. 642) failed to give outstanding results when compared to electroplated tubes that were not terne coated. While isolated anomalous results are frequently obtained in endurance or longevity type testing, tube No. 642 probably had a surface or seam defect which accounted for its poor performance. The difference in the general level of results reported in Table 111 as compared with those of Table 11 may result from unintentional differences in the processing of the samples. In any event, the results of Table 111 would indicate that an electroplated primary coating covered with a secondary hot applied terne coating will give consistently superior results when the primary coating is at least 0.0004 inch thick. 1n the case of primary coatings consisting of nickel over electroplated copper, as thin a coating as 0.0003 inch appeared to give greatly improved results. The most consistently superior results, however, were achieved when the electroplated primary coating was of a minimum thickness of 0.0007 inch.  
  In all samples tested the total electroplated coating did not exceed 0.002 inch in thickness. Electroplated nickel coatings of 0003-0005 inch thick over copper, for example, are typical when superior corrosion resistance is desired. However, the cost of applying such a thick coating of nickel or other expensive metal to steel tubing would make the cost ofthe entire tubing so high as to be undesirable. Accordingly, all experiments were conducted with relatively thin electroplated layers of not more than 0.002 inch in total thickness. The pres ent invention is distinguished by the discovery that a&#39; relatively thin plating of nickel and/0r copper can be made highly effective in protecting a steel substrate from corrosion when covered by a terne metal applied molten. Accordingly, the present invention contemplates the use of a primary coating consisting of one or more electroplated layers composed principally of nickel and/or copper. The thicknesses of such primary coating is broadly within the range of about 0000-00019 inch and preferably 00007-00015 inch in thickness. While the primary coating may be applied in one layer or several layers, it preferably includes a layer of nickel applied over a layer of copper, in which case the total primary coating thickness may be as thin as 0.0003 inch. The present invention further contemplates the use ofa secondary coating terne metal which is applied by passage through molten terne, the thickness of the terne coating being broadly in the range of 9 0.05-0.30 mil and preferably 0.15-0.25 mil in thick ness. Coatings in these ranges are shown in FIGS. 4, 5 and 6.  
  FIG. 2 shows in exploded form a typical arrangement of coating layers applied to the tube outer surface 16 in accordance with one form of the present invention. This particular combination of layers corresponds to sample tube No. 375 of Table I. As will be seen from FIG. 2, a 0.00036 inch thick layer of copper I8 is electroplated on the surface 16 and a 0.00036 inch thick layer of nickel 20 is electroplated over the copper I8. The tube 10 is then passed through a hot terne metal bath to produce a 0.00018 inch thick layer of terne metal 22 over the nickel 20. The copper l8 and nickel 20 comprise two layers of what may be termed a primary electroplated coating while the terne metal 22 comprises a secondary coating overlying the primary coating.  
  FIG. 3 schematically illustrates one method for continuously processing a tube 10 (or a plurality of such tubes) to produce the coatings of FIG. 2. Copper brazed steel tubing is frequently made in long lengths of approximately I feet. A plurality of tubes of such lengths may be processed in side by side relation through the apparatus of FIG. 3. It is also possible to join tubes end to end to produce an endless tube. In any case. the ends of the tubes 10 are crimped or otherwise sealed to exclude processing chemicals and other foreign matter from the tube interior.  
  According to the apparatus of FIG. 3, one or more tubes 10 are advanced successively through a cathodic cleaning bath 24 containing sodium hydroxide and sodium cyanide, a wash bath 26, a cathodic cleaning bath 28 of sulfuric acid. a water wash bath 30, a copper strike bath 32, a copper electroplating bath 34, a water wash bath 36, a nickel electroplating bath 38 and a water wash bath 40. The tube 10 is then fed into terne coating apparatus including a hydrochloric acid cleaner 42 and a hot terne metal bath 44 through which the tube I0 is directed by guide castings 46. After leaving the terne metal bath 44, the tube 10 passes through a pneumatic wiper or air die 48 to control the thickness and uniformity of terne metal applied thereto, after which the terne metal coating on the tube 10 is chilled by the passage of the tube-through a water bath 50. Various pairs of drive rollers 52 advance the tube 10 through the apparatus of FIG. 3.  
  While all of the sample tubes referred to in Tables I, II and III were processed by hand to apply the electroplated coating(s) and not in continuous apparatus such as illustrated in FIG. 3, except for the hot terne coating, the sequence of processing steps described in connec tion with FIG. 3 corresponds to Process No. utilized in the preparation of many of the experimental tubes including tube No. 375. In the processing of tube No. 375. the chemicals. times, currents and temperatures in the steps corresponding to the cathodic cleaning bath 24, the cathodic cleaning bath 28 and the copper strike bath 32 were as follows:  
 Sodium Hydroxide Sodium Cyanide Cathodic Cleaning Bath [22) Sodium Hydroxide 200 gm/l Sodium Cyanide 6 gm/l Room Temperature Current per tube 3.75 amps 4 Volts Time l5 seconds -Continued Sodium Hydroxide Sodium Cyanide Cathodic Cleaning Bath (22) Sulfuric Acid Cathodic Cleaning Bath (24) 10% Sulfuric Acid Room Temperature Process No. 4 of Table I, which was used to prepare many of the tubes of Tables II and III also appeared to be desirable. The steps were as follows:  
 PROCESS NO. 4  
 See Tubes No. 373 and No. 374  
 . Cathodic Cleaning: Sodium Hydroxide. Sodium Cyanide Wash Nickel Strike Wash . Copper Strike Copper Plate Wash . Nickel Strike Wash . Nickel Plate Wash  It should be mentioned that during salt spray testing of tubes a pin hole&#34; rust spot may appear on a tube. The point at which the appearance of a pin hole rust spot occurs is noted. If the spot continues to grow. the point in time at which its inception was noticed is regarded as the inception of rust. If, however, a pin hole rust spot does not grow, but remains static, this is not regarded as the inception of rust as it probably resulted from a small ferrous particle occluded in the coating and not from the substrate. Rust from such small occlusions later may disappear entirely during the testing. All of the salt spray tests referred to herein consisted of standard American Society of Testing and materials neutral 5% salt spray tests. This type of test is covered by ASTM Designation: 8117-64.  
  The reasons for the exceptional results achieved with the particular combination of primary and secondary coatings described herein are not fully understood. It is thought that the electroplated coating provides a surface for the terne coating which enables terne coating to be applied with greater uniformity and tenacity than was previously possible. Viewed somewhat differently, it may be that such thin electroplated coatings by themselves have a certain amount of porosity which subjects them to corrosive attack. When the terne metal is applied, the pores are filled to produce a combined coat ing which is non-porous and of great uniformity.  
  The cost of producing copper brazed steel tubing coated in accordance with the present invention is believed to be about 50% more than the cost of producing the same tubing which is terne coated only. In analyzing the incremental cost of the electroplating, it is significant to note that copper is currently 2 /2 to 3 times as expensive as terne metal and nickel is 7 /2 to 8 times as expensive as terne metal. The cost of applying electroplated layers bears a close proportional relationship to the thickness of the layer electrodeposited. It will. therefore, be apparent that a corrosion coating process which permits the use of such thin layers of nickel and- /or copper as are disclosed herein has significant cost advantages over the relatively thick nickel and copper electroplated layers which were heretofore believed necessary in order to achieve any significant degree of corrosion protection for ferrous substrates.  
  It should be pointed out that the coating described herein need not be in direct contact with the ferrous substrate. It is believed possible to use such coatings over various undercoatings or on steel which has been previously coated. For example, the coating of the present invention may be applied to a previously galvanized steel or a steel to which a preliminary plating of zinc has been applied. In that case, the zinc might perform much of the function of the copper in the previously described copper-nickel combination primary plating.  
 What is claimed is:  
  l. The method of making steel tubing which includes the steps of providing a steel strip havin&#39;g copper electroplated on the opposite sides thereof, rolling said strip into tubular form, furnace brazing said strip to bond and seal the tube, electrodepositing on the outer surface of the tube a primary coating consisting of one or more layers of the metals copper and nickel having a total thickness of between 0.0003 and 0.0019 inch and thereafter coating the tubing with hot terne alloy without intermediate heat treating sufficient to cause appreciable diffusion of said primary coating into the outer steel surface of said tubing.  
  2. The method set forth in claim 1 in which said primary coating includes an electroplated layer of copper and an electroplated layer of nickel applied over the copper.  
  3. The method set forth in claim 1 in which said primary coating is applied to a thickness of 0.0007 to 0.0015 inch.  
  4. The method set forth in claim 1 in which said primary coating includes an electroplated layer of copper and an electroplated layer of nickel applied to a total combined thickness of 0.0007 to 0.00l5 inch.  
  5. The method of forming a corrosion resistant coating on steel tubing which includes the steps of continuously feeding a long length of steel tubing through a series of baths including a cleaning bath, at least one electrolytic bath operable to deposit on tubing outer surface a primary coating consisting of one or more of the metals copper and nickel and having a total thickness of between 0.0003 and 0.0019 inch and a hot terne alloy bath downstream of said at least one electrolytic bath, the movement of the tubing from the electrolytic bath to the hot terne alloy bath being continuous and without heat treatment sufficient to produce any significant diffusion of the primary coating into the outer steel surface of the tubing.  
  6. The method set forth in claim 5 in which said at least one electrolytic bath includes a copper bath and a nickel bath downstream of said copper bath.