two variables are significant to development of specks or hairline cracks in machined engine cylinder barrels after black oxide treatment : the chemical composition of the black oxide bath mixture and residual stresses produced by machining . the standard industry method used to place an oil - retentive black oxide coat on the surface of an aircraft engine cylinder barrel , among other industrial uses , employs a bath solution of sodium hydroxide ( naoh ), sodium nitrate ( nano 3 ), and sodium nitrite ( nano 2 ) in a standard ratio of 80 / 10 / 10 . the 80 / 10 / 10 solution is prepared by dissolving in a gallon of water 5 . 5 lb premixed , dry naoh , nano 3 , and nano 2 which are combined so that the ratio of naoh / nano 3 / nano 2 in the dry mix is 80 / 10 / 10 by weight and the resulting solution boils at 285 - 290 degrees fahrenheit . we performed the following series of experiments to determine the effect , if any , on the observed amount and extent of specks or hairline cracks of varying the composition of the solution . metallurgical testing . for metallurgical testing , high order machined cylinder barrel test pieces were treated in a black oxide chemical bath containing a solution composed of : 80 % naoh / 10 % nano 3 / 10 % nano 2 ( 80 / 10 / 10 ); 60 % naoh / 20 % nano 3 / 20 % nano 2 ( 60 / 20 / 20 ); 80 % naoh / 20 % nano 2 ( 80 / 0 / 20 or 80 / 20 ); 60 % naoh / 10 % nano 3 / 30 % nano 2 , ( 60 / 10 / 30 ); 60 % naoh / 40 % nano 3 ( 60 / 40 / 0 ); and 60 % naoh / 40 % nano 2 ( 60 / 0 / 40 , or 60 / 40 ). these solutions can readily be prepared as follows . the 60 / 20 / 20 solution , for example , is prepared by dissolving in a gallon of water 7 . 0 lb premixed , dry naoh , nano 3 , and nano 2 which are combined so that the ratio of naoh / nano 3 / nano 2 in the dry mix is 60 / 20 / 20 by weight and the resulting solution boils at 285 - 290 degrees fahrenheit . the 60 / 40 solution is likewise prepared by dissolving in a gallon of water 7 . 5 lbs premixed , dry naoh , nano 3 , and nano 2 which are combined so that the ratio of naoh / nano 2 or naoh / nano 3 in the dry mix is 60 / 40 by weight and the resulting solution boils at 285 - 290 degrees fahrenheit . for each bath solution , the cooling fin roots from cylinder barrel number 101 were analyzed for the presence and location of specks and / or hairline cracks . the results of this study are shown in fig1 - 6 . as shown in fig1 test samples treated with the industry - standard 80 / 10 / 10 solution showed pronounced specks up to 0 . 00006 in . in depth . generally , as shown in fig2 and 3 , no specks were observed in test samples treated with the 60 / 20 / 20 and the 80 / 20 solutions , but a few localized areas did exhibit small specks up to 0 . 00003 in . deep . as shown in fig4 test barrels treated with the 60 / 10 / 30 solution exhibited specks up to 0 . 00003 in . deep in a few less localized areas . as shown in fig5 a black oxide bath solution containing 60 % naoh / 40 % nano 2 most effectively prevents specks and hairline cracks . test samples treated with 60 % naoh and 40 % nano 2 showed substantially no specks or hairline cracks in the cooling fin roots . as shown in fig6 many hairline cracks up to 0 . 00038 in . in depth were observed in test samples treated with 60 % naoh and 40 % nano 3 . once we determined the chemical composition of black oxide that was most effective in reducing observed specks or hairline cracks caused by caustic stress corrosion cracking , we compared black oxide coating characteristics — oil retention , abrasive resistance , corrosion , break - in , and the color of cylinder barrels treated in the different black oxide solutions to those obtained in cylinder barrels treated with the standard 80 / 10 / 10 black oxide bath . x - ray diffraction tests were performed to confirm the coating formed during the black oxide treatments , and the result was used to reconfirm the mechanism described previously . cylinder barrel panels were bathed in five different black oxide bath solutions , namely 80 % naoh / 10 % nano 3 / 10 % nano 2 ( 80 / 10 / 10 ); 60 % naoh / 20 % nano 3 / 20 % nano 2 ( 60 / 20 / 20 ); 80 % naoh / 20 % nano 2 ( 80 / 0 / 20 or 80 / 20 ); 60 % naoh / 10 % nano 3 / 30 % nano 2 ( 60 / 10 / 30 ); and 60 % naoh / 40 % nano 2 ( 60 / 0 / 40 ). for these tests , one hundred 1 in .× 4 in .× 0 . 065 in . test panels ( unless otherwise specified ) were prepared either from a rectangular 1 in .× 4 in . sae 4140 bar stock or a 4 . 5 in . diameter round bar stock . panels were divided into five test groups with twenty panels per group . test panels were nitrided and honed on both surfaces for the oil retention , corrosion , and abrasion analyses ; only one surface was nitrided and honed for the color and x - ray diffraction analyses . following black oxide treatment , all test panels were treated with rust - preventive metalguard 450 oil . for testing , the oil was then removed by soaking in mineral spirits for five to ten minutes followed by acetone rinse . taber abrasion tests were conducted by ithaca materials research testing laboratory ( imr , lansing , n . y .). black oxide - coated samples were tested on a taber abraser according to specification astm d - 4060 with 500 g weights and cs - 10 wheels . twenty - five cycles was determined to be an appropriate test length for these thinly coated samples . results are shown in table 1 . although test results were inconsistent due to the thin coating of oil on the panels , we conclude that the black oxide coating achieved using a 60 / 40 solution had slightly inferior abrasion resistance compared to that using the standard 80 / 10 / 10 solution . the test group treated with the 80 / 10 / 10 solution exhibited the best average abrasion resistance ( 0 . 0058 g average for both sides ). the 60 / 40 mixture group had an abrasion resistance ( 0 . 0100 g average for both sides ) slightly worse than that of the 80 / 10 / 10 group . the test group treated with the 80 / 20 solution exhibited the worst abrasion resistance ( 0 . 0150 g average for both sides ). because black oxide coating provides only minor corrosion protection , hubbard - hall , inc ., waterbury , conn ., conducted the corrosion tests in a humidity chamber per astm d - 2247 rather than in a salt spray chamber per astm b - 117 . black oxide - coated panels were soaked in aquaease pl 72 - a32 , cleaned in methylene chloride to remove any residual oil , and then placed into the humidity chamber . the test results , expressed in terms of time before initial rust development , are summarized in table 2 . although the overall test results did not reveal any measurable difference in the groups &# 39 ; resistance to rust , the 60 / 40 black oxide solution group exhibited the best rust resistance , averaging 174 . 8 hours before rust began to develop . no standard test methods or specifications were available to analyze oil retention . thus , ithaca materials research testing laboratory ( imr , lansing , n . y .) used the following method . samples were soaked in iso - octane for two minutes , dried , and weighed . they then were immersed in textron lycoming engine oil for 30 seconds , allowed to drain for 30 seconds , placed between a lint - free cloth and a 4 . 5 lb rubber - faced roller , and the roller rolled over the samples five times . this process was repeated using a new cloth , and the samples were weighed . any weight gain was assumed to correlate to retained oil . results obtained are shown in table 3 . there was no noticeable difference in oil retention properties between the sample groups . somewhat lower oil retention was measured with the 60 / 20 / 20 , 60 / 10 / 30 , and 80 / 20 solutions . however , the average weight gain measured was lower for the 60 / 40 solution ( 0 . 0366 g ) than for the industry - standard 80 / 10 / 10 solution ( 0 . 0368 g ). color testing of the treated panels was conducted at hunter laboratory , reston , va ., and funded by hubbard - hall , inc ., waterbury , conn ., according to specification astm d - 2244 using an ultra scan xe spectrophotometer which measures visual wavelength ( 380 - 750 nm ) photometric response and then records the reflectance . color is measured in terms of lightness ( l ) and tint ( a and b ). for example , when l = 0 , a = 0 , and b = 0 , the color is perfect black without any tint . l varies from 0 , perfect black , to 100 , perfect white . any color in the spectrum can be expressed in terms of the l , a , and b parameters : “+ a ” means red tint , “− a ” means green tint , “+ b ” means yellow tint and “− b ” means blue tint . the major compound formed on the surface coating of the panels following black oxide treatment is magnetite , fe 3 o 4 , which appears black . some fe 2 o 3 is present as well , which appears red - brown to black . the varying amount of fe 2 o 3 on panel surfaces appears to cause the slight color differences . test results are summarized in tables 4a and 4b for the convex side , nitrided surface and the concave side , non - nitrided surface , respectively . there were some measurable differences in color between the nitrided and non - nitrided surface . non - nitrided surfaces were more brownish and darker in color and tint . panels treated with the 60 / 40 solution were more brownish with the same degree of black when compared to the industry - standard 80 / 10 / 10 solution . x - ray diffraction tests were conducted by lambda research , cincinnati , ohio . coatings on the panels were removed by placing the panels in toluene for at least ten minutes and then rinsing them with acetone while they were still wet from the toluene . x - ray diffraction patterns were obtained using graphite monochromated copper k - alpha radiation on a computer - controlled , bragg - brentano focusing geometry horizontal diffractometer . patterns were analyzed using first and second derivative algorithms , after golay digital filter smoothing , to determine the angular position and the absolute and relative intensities of each detectable diffraction peak . the diffraction pattern obtained was then compared to tabulated patterns in the powder diffraction file published by the joint committee on powder diffraction standards for identification of the phase present using mdi computer search / match software . the results of this qualitative phase analysis and additional information about the phases identified on each surface are presented in tables 5a and 5b for the convex side , nitrided surface and concave side , non - nitrided surface , respectively . the main phase formed during black oxide treatment in all solutions was fe 3 o 4 or fecr 2 o 4 spinel phase , on both the convex side , nitrided surface and concave side , non - nitrided surface of the panels . the fe 2 o 3 rhombohedral oxide phase also present on both surfaces most likely occurred as a by - product result of the black oxide treatment . test results therefore show nearly no specks and no hairline cracks in cylinder barrel cooling fin roots subjected to a black oxide bath solution composed of 60 % naoh and 40 % nano 2 . this solution preserved the oil retention , color , scuff resistance , and anti - corrosion properties of the industry - standard 80 / 10 / 10 solution . engine tests were performed to determine the effects , if any , of the new black oxide coating on engine break - in characteristics . actual cylinder barrels treated using the standard 80 / 10 / 10 and new 60 / 40 black oxide bath were tested in the actual engine . more specifically , two cylinder assemblies , p / n 16a22130 - ya , in which the barrels had been black oxide treated in the 60 / 40 bath were included in test runs on io - 360 - a1b6d engine , s / n l - 959 - x at cylinder locations no . 1 and 4 . for comparison , two standard cylinder assemblies , p / n 16a22130 , were included in engine locations no . 2 and 3 . the total accumulated test time was 164 hours . a special run - in test was performed for the first twelve hours to investigate the break - in characteristics of the black oxided cylinder barrels . testing then was continued for an additional 152 hours to build the endurance test time . the cylinder barrel dimensions , average cylinder barrel surface roughness ( intake side and exhaust side ), piston pin plug dimensions , as well as piston ring gaps and tensions were measured after 6 , 12 , 79 , and 164 hours of running time . the resulting test data revealed no significant difference in the break - in characteristics of cylinders black oxide coated in 80 / 10 / 10 and 60 / 40 black oxide baths . no significant differences in cylinder barrel wear , piston plug wear , piston ring gaps , and cylinder barrel roughness were observed . no specks or hairline cracks are visible in cylinder barrels until after black oxide treatment . it is known that residual stress in the surface of cylinder barrels from machining causes specks or hairline cracks to occur by caustic stress corrosion cracking when such barrels are subjected to the black oxide bath . the possibility of hydrogen diffusing into the metal has been reduced , if not eliminated , by increasing the nano 2 in the bath so that the development of specks or hairline cracks can be prevented . we also have found that the lower the residual stresses from machining , the lower the probability that specks or hairline cracks will occur during black oxide treatment . thus , it appears that the extent of the specks or hairline cracks visualized on cylinder barrels also may depend , at least in part , on the amount of residual stresses induced by machining present in the cylinder barrel . the maximum number of cylinder barrels free of specks or hairline cracks after black oxide treatment therefore determines the maximum number of cylinder barrels to be machined on a given set of tool bits . while the invention has been described in connection with exemplary embodiments thereof , it will be understood that many modifications in both design and use will be apparent to those of ordinary skill in the art , and this application is intended to cover any adaptations or variations thereof . therefore , it is manifestly intended that the claims and the equivalents thereof only limit this invention .