I N D CSTRIAL A X D E.VGINEERISG CHEMISTRY
352
Schenectady. To date the films are in excellent condition. Further tests are in progress, iticluding a number upon wood.
A comparison of the characteristics of paint, nitrocellrtlose lacquer, and air-drying resin films is giveii in Tahle 111. Conclusion
The film-forming cliaracteristics of tlie alkyd resins are important to tlie paint and varnish chemist. Three types of alkyd resins can be. prepared differing in their conversion to a non-fusible, jelly-like state, namely(1) heat-noli-convertible, (2) heaGeonvertible, and (3) oxygenconvertible. Each produces resin solutiolis that yield films with singular
7-01. 21, No. 4
characterist,ics. In gemral, we can divide these solutions into (1) baking, and (2) air-drying. The former require heat to develop their inaximuni properties, outstanding of which are touglmess, adhesiveness, flexibility, oil resistance. The latter require primarily reaction with oxygen, although lieat can also be used, in which case its function is essentially to speed up tlie oxygeti reaction. These air-drying films possess t.lie same outstanding properties as the films from the baking solutions, t,ogcther with an additional pronounced film-building characteristic. The air-drying solutions can in a sense be considered as truly synthetic varnishes, as they simulate, not only physipally and ineclianieally, but chernically as well, the Mends from the old-time varnisli kettle.
Corrosion-Note on an Apparent Relation of Protective Film to Microstructure'
T HAS been pointed out2 that the concept of protective film format,ion should play an important part in the d e velopment of corrosion-resistait alloys. This paper Dresents a case illustrative of one possible aspect of such a11 application. The case wax oiie of corrosion in a small gear piiinp used to force a sodium xmtliate solution of cellrrlose ilito the spinnerets of a rayon mill. The gears were of rhrome-manganesc steel with 0.2 per cent carhoii and rotated between plates of cast iron having the following analysis: silicon 2.20, sulfur 0.10, manganese 0.50, phosphorus 0.55,total rarhon 3.50 per cent. With certain import a n t exceptions, all parts of the apparat.iis cirming into cont,art with the solution showed a tliin but dense and a d h e r e n t eonting of ferrous sulfide, which clearly had r o n s t i t u t , e d an efficient protective film a g a i n s t progressive corrosion. The cxceptions were t h o s e portions of t,he cast,iron sideplat,es a d jacent to the periphe r i e s oF tlie g e a r s . These showed strong corrosion, the iron lieing roughened. The site of this action is also the site of the greatest relabive movement b e twe eii these parts, and the
I
I Received December 21. 1928.
whit
m o . . a,
419
a " , Chern.
(iwmi.
Figure I-Segment of Cast-Iron Side Plate Showin8StrongCorraalon Adjacent to Perloher) of Revolving Gear. 3 x
logical conclusion is that the protective sulfide film had liere bcen continoously slicarcd or rubbed off, the rather sharp dividing line between tlie pit,ted circumferential zone and the inner relatively unaffectcd portion indicat,ing the point at which the rate of shear became greater tlian tlie film could resist. (Figure 1) Especially notable is tlie fact tliat the opposing faces of the germ tliemselves showed an effective film and no corrosion. The same was true (if the periphery of the gears and t,he upposing surfaces of the housing, between which the clearance v a s extremely close. Unlike the side plates, however, these parts were made of high-grade steel as indicated aboi-e. The microscope revealed a t once that the roughened or pitted apyearance here was due to attack along the grain boundaries, leaving the grains tliernselves in strong relief. (FiLqre 2) This clearly suggests that the segregation of impurities along grain boundaries characteristic of cast iron was i n some way primarily responsible for the destruction of the protectivc film, prubably hg causing coincident lines of weakness in the film and thus Facilitating rnecliariical (lisruptioii. On this hypot,hesis s e g r e g a t i o n along grain boundaries may play a vital part in the resistance of alloys in general to combined riirrosive a n d mechanical attack, and may aceoitnt fur the effect of r e l a t i v e l y minute percent,agesof c o m p o n e n t s . The e f f e c t m a y be coiis t r i i c t i v e q u i t e :is easily as destructive, FWure 2-Corraded Area in Figure 1 s m ~ ethe segregated Enlarged, Showing Aireck along Grain matter can be thought ~ ~ ~ ~ d ~ r i ~ of as forming a resistant as well as a iion-resirtant deposit against chemical attack.
