Floating rust preventing film coated steel members for buildings and structures
United States Patent [191 Watanabe et a1.  
 1451 Apr. 15, 1975 [22] Filed:  
 [ 1 FLOATING RUST PREVENTING FILM COATED STEEL MEMBERS FOR BUILDINGS AND STRUCTURES [76] Inventors: Shozo Watanabe, 12-66,  
 Komatsu-cho, Hirohata-ku; Minoru Kitayama, 3-29, Azuma-cho, Horohata-ku; Shigenori Yamadera, 32, Kitagawara-cho, Hirohata-ku; Yasuyuki Taniguchi, 14, Katsuhara-cho, Katsuhara-ku; Susumu Yamaguchi, 4-85, Kamiya-cho, all of l-limeji, Japan May 6, 1974 211 App]. No.: 467,566  
 Related US. Application Data [63] Continuation-in-part of Ser. No. 197,440, Nov. 10,  
 1971, abandoned.  
 [52] US. Cl 428/336; 148/6.14 R; 148/315;  
  428/458; 428/461; 428/463; 428/472 [51] Int. Cl B32b 15/08; B44111 1/36 [58] Field of Search 117/132 B, 132 C, 133,  
 117/161 K, 161 UZ, 161 UC, 161 UP, 162; 148/6.14R,31.5  
 [56] References Cited UNITED STATES PATENTS 2,148,862 2/1939 Kern 117/132 C 2,238,651 4/1941 Kcencn.... 106/14 UX 3,249,447 5/1966 Higgins 106/14 3,502,512 3/1970 Bigos et a1. 148/635 3,617,364 11/1971 Jarema 117/132 BE FOREIGN PATENTS OR APPLICATIONS 712,902 8/1954 United Kingdom 252/387 OTHER PUBLICATIONS Belyi et a1., Chem. Abs. Vol. 72, No. 15285g, Jan. 1970.  
 Maksimov et al., Chem. Abs. V01. 68, No. 62070k,  
 Primary Examinerl-1arry J. Gwinnell Attorney, Agent, or FirmWenderoth, Lind &amp; Ponack [57] ABSTRACT Steel members, particularly weatherproof low alloy steels for use in buildings and structures, the surface of said steels being coated with a film containing an oxide pigment mainly composed of manganese dioxide and the rest being a film forming composition for the purpose of preventing the formation of an undesirable floating rust on said surface while promoting the formation of a stable hard protective rust on said steel surface.  
 5 Claims, 3 Drawing Figures PATENTEUAPR 1 51975 3,877,  
 SHEET 1 BF 2 PIIIEI-ITEIIIPR I 5I975 3,877. 977  
 SIIEEIEIIfZ VARIATION IN THE OUTFLOW OF F6 WITH THE LAPSE OF TIME I NIgI G L RUST INITIAL FLOATING 6O RUST FORMATION F K FORMATION Q ALPHABETS ARE SIGNS OF SAMPLE S OUTFLOW AMOUNT OF Fe (mg/IOOm I.)  
  f UNTREATEO IBBARE) STEEL MEM ER 0 I I I -5 8I0 0 ;E OV R 8% I-Ti/ I I I I 3 6 9 Q I8 2I 24 27 3o 33 36&#39; INVENTOR s EXPOSURE TEST RERIOO (IN MONTHS SHOZO WATANABEI MINORU KITAYAMA, SHIGENORI YAMADERA, Fl YASUYUKI TANIGUCHI AND SUSUMU YAMAGUCHI BYIDQAIIIMM W \LPMOIL ATTORNEY FLOATING RUST PREVENTING FILM COATED STEEL MEMBERS FOR BUILDINGS AND STRUCTURES This is a continuation-in-part of US. application, Ser. No. 197,440, filed Nov. l0, I971, now abandoned.  
  The present invention is directed to the production of steel members coated with a floating rust preventing film to be used for buildings and structures. and more particularly to a weatherprooflow alloy steel to be used for buildings and structures. This low alloy steel is such that it prevents the formation of a floating rust ordinarily formed when conventionally used weatherproof low alloy steels are exposed to the atmosphere. At the same time, the weatherproof low alloy steels of the present invention are such as to promote the formation of a stable rust.  
  lt is well known that a weatherproof low alloy steel has such properties, that, when it is exposed to the atmosphere, there is obtained the formation of a hard compact film or rust on the surface of said steel after the lapse of several years, whereby subsequent corrosion may be prevented. Thus, as the weatherproof low alloy steel shows a weatherproofness several times that of ordinary steel, it is extensively adopted for use in buildings and structures which are particularly exposed to the atmosphere. However, this weatherproof low alloy steel has a shortcoming in that an undesirable yellowish brown rust (which shall be called a floating rust hereinafter) is formed thereupon before the abovementioned hard compact protective rust is formed. Such a floating rust causes various problems in that it flows easily out of the steel surface or peels off after the steel members are piled, thereby not only causing a wasteful consumption of the steel members but also staining the surrounding of buildings and structures made of these steel members.  
  An object of the present invention is to solve the various troubles caused by the generation of floating rust while at the same time promoting the formation of a hard compact protective rust on the surface of the steel. 7  
  The above-mentioned object is attained fundamentally by the following subject matter according to the present invention.  
  The said object of the present invention can be achieved by coating the surface ofa-weatherproof low alloy steel with a film consisting of an oxide pigment, including manganese dioxide, and a film forming component. In this case, it is immaterial whether the film may be formed on the steel surface in the step before or after the steel member is assembled in a structure or building.  
  FIG. 1 is a photograph showingthe, state of a steel member being stained in the surroundings and consumed by the outflow ofa floatingrust produced before a stable rust was formed when a bare weatherproof low alloy steel was exposed to the atmosphere.  
  FIG. 2 is a photograph showing the surface state when a weatherproof low alloy steel treated according to the present invention was exposed to the atmosphere for about 9 months.  
  H65. 3 and 4 are photographs showing states of a compact stable rust formed on a steel surface after the lapse of about months and about 18 months respectively, when a weatherproof low alloy steel treated according to the present invention was exposed to the atmosphere.  
  FIG. 5 is a graph showing the variation ofthe amount of iron flowing out of the surface of a sample during an exposure test period.  
  The floating rust preventing film according to the present invention consists of an oxide pigment, including manganese dioxide, and a film forming component so that, when a weatherproof low alloy steel is coated on the surface with such film (which shall be called a floating rust preventing film hereinafter), water may be inhibited from penetrating the surface ground of the steel from outside by the action of the film forming component. The oxide pigment contained in the film, on the other hand, forms a bridge for oxygen in a stable rust accelerating film while, at the same time, acting as a carrier for oxygen due to the catalytic action of the manganese dioxide contained in the same film. whereby oxygen may be fed in turn to the steel surface ground from outside through the film.  
  When the weather proof low alloy steel coated with a floating rust preventing film, which has such operating mechanism as abovementioned, is exposed to the atmosphere, the oxidation of the steel surface ground below the floating rust preventing film is accelerated without passing through the process of an ordinary wet corrosion, that is, without undergoing the formation of a floating rust, because active oxygen (in an atomic form) is fed to the steel surface ground through the film. The penetration of water into the steel surface ground is, however, kept to a minimum by the action of the film forming component (vehicle). Moreover, because the stable rust produced on the steel ground surface is incorporated with a part ofthe oxide pigment contained in the floating rust preventing film, said stable rust becomes more compact. whereby the progress of corrosion is restrained without undergoing the formation of the floating rust.  
  This stable rust shows, for example, a blackish brown or dark brown color, is compact and hard. and has a sufficient weatherproofness. Therefore, it has a performance comparable to that of an ordinary stable rust formed on the surface of a bare weatherproof low alloy steel, which has been exposed to the atmosphere for 4 to 5 years and, after the floating rust preventing film has been peeled by weathering, the stable rust exhibits a corrosion preventing effect.  
  In such case, all the oxide pigment can be made of manganese dioxide only, but in order to improve the tone of the floating rust preventing film, it is desirable to use an oxide pigment consisting of manganese dioxide and one or more of such other oxide pigments as are selected from among for example, iron oxide, copper oxide, titanium oxide and lead oxide. In such case, however, it is necessary that the amount of manganese dioxide should be at least l% and that the oxide pigment including manganese dioxide should be 30%, When the oxide pigment is less than 30%, a bridge for feeding sufficient oxygen to the steel surface ground is scarcely formed in the floating rust preventing film. On the other hand, any oxide pigments other than manganese dioxide are so weak in oxygen carrying capacity that they do not form a stable rust on the steel surface ground. However, if at least 1% manganese dioxide is contained, the oxygen carrying capacity is remarkably elevated by its catalytic action and a stable rust can be formed on the steel surface ground, while a floating rust preventing film is still present on the steel surface.  
  From the viewpoint of the oxygen carrying capacity, the upper limit of the amount of the oxide pigment is desirably high. However, if the content thereof is too high, there are many disadvantages in that the spray nozzles are clogged at the time when the stable rust film is being formed and the stickness and adhesiveness of the film are reduced. In view of these facts,&#39;therefore; the upper limit value of the oxide pigment is desirably about 70%.  
  Next, the thickness of such floating rust preventing film shall be explained. The thickness of the film is intimately related to its performance and life. If the film is too thin, there are produced such film defects as pinholes and painting irregularities. Further, an ordinary wet corrosion is produced on the steel ground surface as well as a stable rust and a floating rust. On the other hand, if the film thickness is too large, there occurs a shrinking phenomenon in the film. When taking such defects and the coarseness of the surface to be painted into consideration. the thickness of the floating rust preventing film is preferably about to 70 ,u.  
  The film forming component to be used as the base of the floating rust-preventing film of the present invention is required to be capable of forming a film during the lapse of 2 or 3 years, after it has closely adhered to the surface of a weatherproof low alloy steel, while holding particlesof the above mentioned oxide pigments dispersed in the film. I  
  The film forming component of the present invention must exhibit the following characteristics in order to meet the above mentioned requirement.  
  First, the film forming component of the present invention is required to be waterproof. Further, it is required to have the property of dry-hardening at a normal temperature (a thermo-hardening property is not practical because the film is to be applied to buildings and structures, according to the present invention. Still further, the film forming component must have excellent adhesiveness to the descaled surface of the weatherproof low alloy steel, as the film must be held on the surface of steel materials for use in buildings and structures. and also, the film must be adhesive so as not to peel off when the steel materials are assembled and worked.  
  Since the floating rust preventing film of the present invention should be replaced by a stable rust formed under the said film after a lapse of 2 or 3 years, the film forming component is not required to have a durability of more than 3 years to exposure in the atmosphere; rather it is preferable that the film be weathered after a lapse of 2 or 3 years. a  
  As a result of the inventors search for materials satisfying the above mentioned requirements, it has been discovered that organic resins such as alkyd resins (particularly, oil-modified alkyd resins and styrenated alkyd resins), acrylic resins (particularly, acrylic resins of the emulsion type), phthalic acid ester resins, and cumarone resins, and rubber derivatives such as cyclized rubber, rubber chloride and polyalkylene sulfide resins (Thiokol) are suited to the use as a film forming component which possesses the waterproofness, dry hardening property at normal temperature and close adhesiveness to the steel as above mentioned.  
  Specific examples of the resins contemplated in the present invention will be described below.  
 Acrylic Acid Ester Resins These are resins obtained by the copolymerization of acrylates under the action of a catalyzer such as benzoyl peroxide and the, like, said acrylates having carbon atoms usually within the range of 1-4 in the alkyl group, forming the ester moiety (i.e. the residual alcohol group) with the acrylic acid.  
  The physical and chemical properties and the trade name of the acrylic acid ester resin used in the experiments of the present invention are as follows:  
 Specific gravity 1.3 l.5  
 Tensile strength (kg/cm) l00 l 10 Elongation (/z) 400 500 Modulus of elasticity ltlkg/cm&#34;) I 220 Water-absorbing percentage (Dipping for 24 hours) (7:) 0.3 0.4  
 Viscosity (of 25% toluene solution at 25C) (Stokes) 3 4 Dipping in l071 H 80, at 25C X 240 hrs. Swelling Dipping in 10% NaOH at 25C X 240 hrs. Swelling Additive No addition Trade name (Manufacturer) Alon S 1002 (TOA GOSEl CO..  
 Japan) Tensile strength (kg/cm&#34;) 425 490 Elongation (71) 1.5 2.5  
 Modulus of elasticity (l()&#39;kg.cm&#34;&#39;) 3 3.5  
 Water-absorbing percentage (Dipping for 24 hrs) (/1) 0.03 0.04  
 Viscosity (of 25% toluene solution at 25C) (Stokes) 4 5 Dipping in 10% H at 25C 240 hrs. No change Dipping in 107: NaOH at 25C X 240 hrs. No change Additive Not known Trade name (Manufacturer) Styren 666 (ASAHl-DOW CO&#34; Japan) Phthalic Acid Ester Resins This is an oil-modified alkyd prepared by modifying a condensate of phthalic anhydride and glycerine by soya bean oil. This resin is composed of 24-26% phthalic anhydride 54-58% of an oil substance and is used as a conventional varnish. Naphthene lead is added as an additive prior to the&#39;use of the resin.  
  The physical and chemical properties as well as the trade name of the resin used in the experiments of the present invention areas follows:  
 Specific gravity l.0l l.2() Tensile strength (kg/cm&#34;) 70 Elongation 1 I60 210 Water-absorbing percentage (Dipping for 24 hrs) (&#39;71) 0.06 0.09  
  -Continued Viscosity (4571 dissolved as nonvolatile matter in toluene) (Stokes) 12.5-14.0 Dipping in l07r H SO at 25C X 240 hrs. No change Dipping in NaOH at C X 240 hrs. Slightly corroded Additive (Naphthene lead) Trade name (Manufacturer) 0.4 0.6 (as lead) Phthalic acid resin which is a vehicle of &#34;TAlKO-MARINE&#34; of DAl NIPPON TORYO. CO. (Japan) \\&#39;as used.  
 Cumarone Resins These resins are obtained by polymerizing a hydrocarbon oil, which has a boiling point range of 130-200C in respect to related coal dry distillation components. at 40-l20C under the existence of a Friedel-Crafts type catalyzer.  
  This resin has the following physical and chemical properties. The resin. however. used in Applicants experiments was self-manufactured.  
 V type Cumaron resin (NIPPON STEEL CO.)  
 Trade name (Manufacturer) Cyclized Rubber This is an isomer of natural rubber and is a rcsino&#39;td obtained by cyclizing natural rubber by means of the action of stannic chloride.  
  The physical and chemical properties and the trade name of the rubber used in the experiments of the present invention are as follows:  
 Specific gravity 1.0 l.l  
 Refractive index nD 1.542 1.546 Chlorine part (71) 1.5 1.7 lodo value 180:10 Water-absorbing percentage (Dipping for 24 hrs) (/11 0.04 0.05 Viscosity (of 25% toluene solution at 25C) (Stokes) 3.5 4.5 Dipping in 1071 H 50, at 25C for 240 hrs. No change Dipping in [U /1 NaOH at 25C for 240 hrs. No change THERMOLlTE P (SEIKO KAGAKL&#39; CO.. Japan) Trade name (Manufacturer) Rubber Chloride Specific gravity 1.5 1.7 Refractive index nN l.55 l-l.555 Tensile strength (kg/cm&#34;) 300-400 Elongation (7r 3 4 Water-absorbing capacity (Dipping for 24 hrs) (/11 0.2 0.3 Viscosity (of 25% toluene solution at 25C (Stokes) 3 4 Dipping in 1092 H 50, at 25C x 240 hrs. No change Dipping in 1071 NaOH at 25C X 240 hrs. No change ALLOPRENE R 20 Trade name (l.C.l. CO.. England) Polyalkylene Sulfide This is a rubber of the polysulfide series produced by the condensation ofa symmetrical alkyl dihalide and an alkali polysulfide such as sodium polysulfide. and is known under the trade name of Thiokol.  
  The physical and chemical properties as well as the trade name of this rubber used in the experiments of the present invention are as follows:  
 Specific gravity 1.2 1.3 Average molecular weight 5000 6000 Properties Sticky fluid at room temperature Viscosity (27C) 350 450 poises Mutual solubility with various solvents Very good solubility with toluene. chlorobenzene and cyclohexan at any ratio.  
 Fairly good mutual solubility with methylethyl ketone. carbon tetrachloride and the like.  
 Bad mutual solubility with methanol.  
 kerosene and the like. &#34;THIOKOL&#34; P 2 (THlOKOL CHEMICAL CO.. USA.)  
 Trade name When using this rubber as a film-forming component. lead dioxide is used as a hardening agent.  
  A paste-formed hardening agent supplied by the THl- OKOL CHEMICAL CO. as an addition agent is admixed at a ratio of 7.5 wt. parts to wt. parts of the LP 2.  
  Conventional paint films generally applied to steel members are mostly to prevent rusting. For this purpose there is preferably used a film composed of a paint-film forming component (vehicle) for preventing the penetration of water and gases (air and corrosive gases) and pigments which function as an inhibitor as, for example. lead oxide. red lead oxide (Pb- 0 or zinc chromate. However. when conventional rust preventing paint is applied to the surface of a weatherproof low alloy steel, and the painted steel is exposed to the atmosphere. the rusting of said steel is indeed inhibited. However. after several years of exposure to the elements. a floating rust is produced on the surface of the steel. Thus. the conventional paint films only serve to delay the generation of the floating rust.  
  According to the present invention. it is very advantageous to incorporate sulfur (S) into the floating rust prevention film. Such S is derived from such sources as sulfur. sulfur compounds and sulfurous substances. and one or more of these substances can be used herein. It is well known that S is an element which accelerates corrosion. However. as a result of making exposure tests of many weatherproof low alloy steels and investigating floating rust preventing films for many years the present inventors have discovered that- S. which has been heretofore considered to be a detrimental element in that it accelerates corrosion and oxidation. may contribute greatly to the formation of a compact stable rust. and this is effected by the reaction ofS contained in the floating rust preventing film with the steel ground surface. In such case. more than 0.05% S is preferably used in the floating rust preventing film and may be used in theform of at least one substance selected from sulfur. sulfur compounds and sulfurous substances. That is. when the S content is less than 0.05%. there is found substantially no effect as a result of the addition of S. in that the films are the same as if no S had been added. On the other hand. if the S content is more than 0.05% a more compact&#39;stable rust is formed much earlier than in the use of a floating rust preventing film which contains no S. However. if the S content is excessive. the weldability of the steel is impaired and the film forming solution itself is deteriorated. From this point of view. the upper limit of the S tive amounts. said solutions being prepared by using a normal-temperature dry type acrylic ester resin as a vehicle (specific gravity: 1.4; viscosity: 3.4 stokes-(2571 toluene solution at 25C); trade name: Alon S 1002).  
  The results shown in the table and figure are obtained by applying said solutions on the surfaces of test pieces of weatherproof low alloy steel (composed of 0.087: C. 0.42% Si. 0.37% Mn. 0.094% P, 0.041% S, 0.20% Ni. 0.30% Cu and 0.60% Cr. 50 X 150mm and a thickness of 3.2mm) descaled by shot blasting in thickness of 25 to 30 ,u.. and exposing the thus-obtained film to the atmosphere. Table 1, shows the time when a stable rust was formed on the surface of the exposed piece and the state of the steel surface ground at that time. FIG. shows the variation of the amount of iron flowing out of the sample surface during the exposure test period..  
 as already mentioned. The so called outflow iron amount designates values obtained by an analysis on the amount of Fe flowing out into water flowing down on the surface of the sample. This is achieved by spraying deionized water on the surface of said sample at fixed intervals. 1  
 Table 1 Pigment in &#34;/1 Time (in months) Surface state at the Sample Oxide pigment in &#39;71 Ebonite Total when a stable rust time mentioned on the Manganese Black iron Total powder in was formed on left No. dioxide oxide is /l of S the steel surface A l 19 20 0 20 No stable rust Yellowish brown floatwas formed ing rust 13 0.5 29.5 0 30 ditto ditto C 1 29 30 0 30 24 Dark brown stable rust D 18 12 30 0 30 28 ditto E 1 29 30 0.04 (0.01 30.04 24 ditto F 1 29 30 0.2 (0.05) 30.2 21 Blackish brown stable rust G 24 16 40 20 5.0) 60 15 ditto H 0.5 59.5 60 0 60 No stable rust Yellowish brown floatwas formed ing rust 1 l 59 60 0 60 24 Dark brown stable rust 1 30 30 6t) 0 60 18 dim K 1 59 60 0.2 (0.05) 60.2 22 Blackish brown stable rust L 30 30 60 0.4 (0.1 60.4 16 ditto content is preferably 10% and the optimum range is 0.1  
 As evident also from Tablel and FIG. 5, it is necesto 5%. For the above-mentioned sulfur compounds. sary that manganese dioxide should be more than 171 there can be used such compounds as sulfates and sulfldes as. for example. CaSO NiSO ZnSO and CuS, and such organic sulfur compounds as ebonite powder, factis and balsam sulfide.  
  The following shows the result of experiments carried out by the inventors.  
 and that the dioxide pigments including said manganese dioxide should be 30%. In case these conditions are not satisfied. there is formed a floating rust. 1f the S to be contained in the film is less than 0.05%. the reaction for producing a compact oxide film rust is not accelerated, and the effect of adding S is not recognized.  
  Concrete examples of the weatherproof low alloy steel as the object to be achieved by the present invention are shown in Table 2. however. the invention is not limited to them.  
 Table 2 Refer- Chemical composition in 7:  
 ence C Si Mn P S Cu Cr Ni V Ti Mo Ni a. 1 $0.12 0.25-0.75 0.20-0.50 0.07-0.15 0.035 0.25-0.55 0.30-1.25 0.65 b. 2 0.10-0.19 0.15-0.30 0.70-1.25 $0.035 $0.035 0.25-0.40 0.40-0.70 0.02-0.10 c. 3 0. I 2 0.25-0.75 1.20 0035-0. 1 2 0.035 0.25-0.50 0.40-10 0.65 0.15 d. 4 0.18 0.35 1.4 0.04 0.04 0.25-0.4 (1.20-0.50 I e. 5 $0.12 $0.60 50.60 0.06-0.12 S 0.04 0.2-0.5 0.4 -1.2 $0.35 f. 6 $0.18 S0.55 51.20 50.040 5 0.04 0.20-0.50 0.4 -1.20 $0.10 50.35 50.10  
 Table 2-Continued Chemical composition in 7: Refer- J y t ence C Si Mn P S Cu Cr Ni V T i Mo N:  
 g. 7 0.15 0.10am filo 5.0.15 0.03, 0.3-0.6 51 $0.65 0.10 0.20 r h. 8 50.]5 $0.60 (lJll-l .0 50.110 0.04 (LI-0.50 0.3041 50.50 T 0.10  
 a. CORTEN o Concrete examples shall be explained in the following:  
 l. Chemical composition of the used weatherproof low alloy steel:  
 0.01071 0.4071 Si. 0.38% Mn. 0.090% P.  
 0.0l77r S. 0.18% Ni. 0.28% Cu and 0.59% Cr.  
  2. Composition of solutions for forming floating rust preventing film:  
  Acrylic ester resin (specific gravity: L4; viscosity: 3.4 stokes (259i toluene solution at 25C: trade name: Alon S 1002) of normal temperature dry type 4092 by weight a) Manganese dioxide l-l&#39;lr by weight Iron oxide (mixed iron oxide of ferrous-ferric oxide and iron scsquioxide 65? by weight Toluene Rest Phthalic acid resin (specific gravity: l.l2: viscosity: 23.7 stokes (45% nonvolatilc resin part dissolved in toluene at 25C): ht trade name: Vehicle of paint TAIKO MARINE&#34;) 40% by weight Manganese dioxide l/( by weight Lead oxide 20); by weight Xylene Rest Styrol resin (specific gravity:  
  l.5: viscosity: 4.3 stokes (25% toluene solution at 25C): trade name (2) Styron 66b) 40% by weight Manganese dioxide l9; by weight Titanium oxide 209&#39;? by weight Copper sulfide I&#39;ll by weight y ne Rest Cyclized rubber (specific gravity: 1.06: viscosity: 4.] stokes (2571 toluene solution at 25C): trade name: THERMOLlTE P) 20% by weight d) Manganese dioxide l4&#39;/( by weight Iron oxide (the same as in a)) 6&#39;71 by weight Ebonite powder (25.47: S) 10% by weight Xylene Rest Rubber chloride (specific gravity: 1.6: viscosity: 3.6 stokes (2571 toluene solution at 2ST]; trade name: ALLOPRENE R 2591 by weight e) Manganese dioxide 1071 by weight Copper oxide 20% by weight Plasticizer (DOP: di-t Z-ethylhexyl phthalatel) 5&#39;1 by weight Xylene Rest painted respectively with the said solutions (a). (b).  
 (c). (d) and (e) by air-spraying. They were then natu rally dried to form floating rust preventing films of a thickness of p..  
  The five samples thus coated with the respective floating rust preventing films were exposed out of doors. A photograph of the state of the surface treated with the treating solution ((1) after the lapse of about 15 months is shown in FIG. 3. As evident from FIG. 3, it is understood that. in about 15 months. the floating rust preventing film peels off and a blackish brown stable rust is formed beneath the said film.  
  The photograph in FIG. 4 shows the state of the surface treated with the treating solution (a) after the lapse of about 18 months. As evident from the same photograph. it is understood that a dark brown stable rust is formed in place of the floating rust preventing film.  
  Further. the samples also treated with solutions (b), (c). (d) and (e) show the same favorable results. though some differences are found in the time when the stable rusts were formed on the steel surface grounds and in the appearance of the obtained stable rusts.  
  The thus obtained stable rusts on the steel surface grounds are all stabilized compact films and do not produce any floating rust. even though they are continuously exposed out of doors. Therefore there is found no staining of the surroundings at all. Also. the corrosion of the steels is very slight.  
  As detailed in the above. according to the present invention. the surface of a weatherproof low alloy steel is subjected to a special coating treatment so that the penetration of water into the steel surface ground may be restricted to a minimum. while the permeation of oxygen may be positively effected, whereby the formation of stable rust on the steel surface ground below the film may be accelerated during the life of the film. Moreover. the formed rust is made stable by that portion ofthe oxide pigment (contained in the floating rust preventing film) which is incorporated into the stable rust. The net result is that the formation of the stable rust prevents the formation of the undesirable floating rust.  
  As a result of the above phenomenon. the staining of the surroundings of buildings and structures made of weatherproof low alloy steel is eliminated. the consumption of the steel materials by corrosion is reduced. and the life of the steel is greatly increased.  
 What we claim is:  
  l. A weatherproof low alloy steel member to be used as a material for buildings and structures. the surface of which is covered with a film for accelerating the formation of a compact and stable rust on the surface of said steel. while preventing the formation of floating rust on the surface of the steel while said stable rust is being formed. said film consisting essentially of at least 1% manganese dioxide; 30-70% of an oxide pigment, or a mixture of oxide pigments. including said manganese dioxide; 0.05 to 10% sulfur; and the rest being an organic film forming component selected from the group consisting of an acrylic resin. an alkyd resin. a  
 consisting of iron oxide. copper oxide. titanium oxide.  
 and lead oxide.  
  4. The weatherproof low alloy steel member according to claim I in which said film on the surface of the steel has a thickness of 15 to u.  
  5. The weatherproof low alloy steel member according to claim 1 in which the film contains 0.1 to 5.0% of sulfur.