Saturday, January 5, 2013

Rust, Part 2 -- car roofs

A vulnerable part of a freight car, as far as weather damage, is the roof. Recognizing this, material for steel car roofs has been galvanized since at least as early as the 1920s, with some installations before that. (In those days, they were outside metal roofs using sheet metal, rather than the “solid steel roofs” of the 1930s and later.) Galvanizing provides another protective barrier, underneath paint, to help prevent rusting of the roof.
     But paint doesn’t always stick well to galvanized surfaces. There are articles from the teens and the ’20s in the professional railroad literature, such as Railway Age and Railway Mechanical Engineer, discussing the problems of getting good paint adherence to galvanized car roofs. The problem, therefore, extends from very near the beginning of steel car roofs until today. But I’m going to talk about my own modeling period, the early 1950s.
     Period photos of prototype car roofs certainly show paint failure on some freight car roofs, which of course exposes the galvanizing underneath. The surface of steel which is freshly galvanized and unpainted has the silvery metallic sheen of the zinc coating, but it fairly quickly oxidizes to a gray color. The zinc oxidizes in preference to any exposed steel (that’s the point of galvanizing), so its oxidation is accelerated relative to what it might be for a piece of pure zinc in isolation. But as zinc continues to oxidize away, eventually the thin layer is consumed and steel is exposed. That of course leads to rusting of the steel.
     This sequence means that very freshly failed paint might show the silvery color of the zinc, but mostly any area of paint failure will look gray. If the patch of gray is large, one can expect a little rust to begin toward the center area of the exposed patch. For modeling purposes, then, I believe silver should be used sparingly and only in quite small areas; that most paint failure should reveal a light gray color; and that only large patches of missing paint ought to show rust. The rust coloration should follow the sequence I mentioned in my previous post (see: ), namely pale yellow for the newest rust, reddish orange for a little bit older rust, and dark brown for any rust which has been exposed for some time.
     Now let me show some examples from my own modeling, as to how this is done. My observation is that paint failure often occurs first at corners and outside angles, so I usually show light failure with that kind of pattern. This gray paint is Floquil “SP Lettering Gray,” a suitably light gray from my perspective, but you can choose any gray you like. (Observing some weathered galvanized steel might be a good guide to such a choice.) Here are two cars with a low degree of failure, shown in front of the depot at Shumala.

Both these box cars have gray only, to indicate minor failures which are not particularly progressing, thus all exposed zinc has turned gray. As I mentioned, most gray areas are at corners or edges in the roof panels.
     Showing more extensive paint failure may only involve more and somewhat larger areas like those shown above. Here are two such cars.

The lower car is a box car, and portrays a difference from the two previously shown: some silver is used along with gray, and a few touches of rust, to indicate ongoing failure. The upper car is a refrigerator car, and shows more extensive paint failure and a little rust near the ice hatches (most of it near one hatch), as could result from spilled salt from the salt additions to ice bunkers.
     Here is a perspective view of another version of this roof treatment, on the Broadway Limited NYC standard steel car. It’s in a train approaching Shumala and is partly hidden by the section house west of town.

     Lastly I show a car with moderately more exposed galvanizing than those already shown. Here some additional weathering was applied to the roof after the gray was applied, to indicate that the paint failure has existed for some time in service.

Here I have tried to portray the paint failure spreading from the places it starts. A little silver is visible at the edges of some gray patches, and touches of rust can be seen also.
     If you want to go further, and certainly there are photos of cars with more extensive paint failure, I think for the 1950s you still want to show primarily gray exposed surfaces under paint. This means the galvanizing is succeeding in protecting the steel underneath, which of course is the whole point. As an example, the photo below was taken by Jim Eager in 1985, of a car built in 1939 (assigned to maintenance service, thus the “AX” prefix on the car number). A fair amount of paint has failed, but the roof is not rusting. There is some rusting elsewhere on the car, however (click to enlarge), and the wood running board is virtually devoid of paint. The photo is used with Jim’s permission.

     Should you model the kind of paint failure I’ve shown in this post on a lot of your freight cars? In my opinion, no. The kinds of yard photos in the 1950s which portray a sea of freight cars show that relatively few cars exhibited paint failure on roofs. I think a model freight car fleet without any paint failure on roofs is inaccurate, but equally inaccurate would be one with lots of roofs in that condition.
Tony Thompson


  1. Soot (especially on coal burners) combining with steam must have formed carbolic acid, with a nice dose of sulfuric acid from the more sulfer rich coal used in the day. The exhaust from a steam locomotive must have been about as bad as acid rain ever got, as it whiffed around and condensed on rolling stock.

    Never the less, as you say, photographic evidence of paint failures are very few. Why? Three theories:

    1. "A protective layer of dirt." Many dismiss this, but I say there is evidence to the contrary. An SD45 that slugs its way through mountain tunnels will have significantly less paint fade, especially in the scarlet red ends, than a switcher. Red paints tend to oxidize and "chalk," developing a white layer that lightens up the paint. Oily diesel exhaust provides a protective layer that prevents the paint from oxidizing. It's true that the soot darkens the tone, but there's a different look (and thus, a reason why you need about 20 shades of SP Scarlet for you diesel fleet). The black doesn't negate the white. It forms gray which effects the saturation of the red. So you've got the white chalking tending to lighten up the paint, the oily soot tending to darken the paint, and the combination of the two (seen on locomotives that may have operated in the sun, say, on the Sunset route, and then assigned to a mountain district, like Tehachapi) where you have a mix of the two effect.

    2. Better paint maintenance. Railroading was much more labor intensive, and companies looked at maintenance as protecting their investment, rather than as a "cost center." This theory held out until the 1960s with the introduction of the Boeing 747, when it became evident that if they maintained it the way other aircraft were maintained, it would never fly. Due to the complexity of it's systems, as soon as maintenance was completed, more maintenance would be due. Enter the Airline Maintenance Steering Group...

    3. Lead in paint acted as a sacrificial anode, providing double protection with the zinc galvanizing to protect the paint. I think most people are shocked to hear of Chinese goods being made with paint containing lead, but this became an issue here in the US sometime in the 1950s. I remember Public Service Announcements on TV advising parents not to allow children to eat paint, and to have the paint in their homes tests. Today we take "lead free" as the norm in our paint, but in third world countries, cheaper older technology still reigns supreme. Like lead paint. Paint "back then" in the steam era did have lead in it.

  2. Thanks, Arved, for generally good comments. But let me add just a bit, since I worked for years in the corrosion world. The dirt layer is a mixed blessing, perhaps shielding paint from fading but furnishing a location which can retain corrodents and moisture.

    And lead is NOT sacrificial relative to iron, but is MORE noble than iron. In effect, the iron is a sacrificial anode to lead! But the individual lead-containing particles are compounds in the paint, not metallic lead, so this does not work anyway.
    Tony Thompson

  3. Darn it - misread my galvanic table. You're right!