There’s a popular myth that I’ve heard about
several bridges (including the Golden Gate Bridge in San Francisco and the Forth Bridge in
eastern Scotland) that they paint the structure continuously from end to end. Once they
finish at one end, they just start back up on the other. It’s not exactly true (at
least for any structures I’m familiar with), but if you drive over any steel bridges
regularly, it might seem like the painting never quite ends. That’s because, despite
its ease of fabrication, relatively low cost, and incredible strength, steel has a limitation
that we’re all familiar with: rust. Steel corrodes when exposed to the elements, especially
when the elements include salty sea air. I’m doing a deep dive series into
corrosion engineering. We’ve talked about the tremendous cost of rust and how
different materials exhibit corrosion, we’ve talked about protecting against rust
using dissimilar metals like zinc and aluminum, and now I want to show you the other major weapon
in the fight against rust. If you’ve ever thought, “This channel is so good, he could make
it interesting to watch paint dry…” well, let’s test it out. I have the rustomatic 3000 set
up for another corrosion protection shootout, plus a bunch of other cool demos as well. I’m Grady
and this is Practical Engineering. On today’s episode we’re talking about high performance
coatings systems for corrosion protection. This video is sponsored by HelloFresh. More on them later. You might have noticed a word missing from that episode headline: “paint.” Of course,
paint and coatings get used interchangeably, even within the industry, but there is a general
distinction between the two. The former has the sole purpose of decoration. For example, nearly
everyone has painted the walls of a bedroom to improve the way it looks. Coatings, on the other
hand, are used for protection. They look like paint on the surface, but their real purpose is
to provide a physical barrier between the metal and the environment, reducing the chance that it
will come into contact with oxygen and moisture that lead to corrosion. Combined with cathodic
protection (that I covered in a previous video), a coating system properly applied and well
maintained can extend the lifespan of a steel structure pretty much indefinitely. Although paint
and coatings often include similar ingredients, are applied in the same way, and usually
look the same in the end, there are some huge differences as well, the biggest one
being the consequences if things go wrong. There are definitely right ways and wrong ways
to paint a bedroom, but generally, the risk of messing it up is pretty small. Sometimes the color
is not quite right or the coverage isn’t perfect, but those are pretty easy to fix. In the worst
scenario, it’s only a few hundred dollars and a couple of days’ work to completely redo it.
Not true with a coating system on a major steel structure. Corrosion is the biggest threat to many
types of infrastructure, and if the protection system fails, the structure can fail too. It’s
not just money on the line, either. It’s also the environment and public safety. Pipelines can
leak or break, and bridges can collapse. Finally, it’s often no simple matter to simply reapply
a coating system because many structures are difficult to access and disruptive to shut down.
Applying protective coatings is something you only want to do once every so often (ideally every 25
to 50 years for most types of infrastructure). That’s why the materials and methods used to
apply them are so far beyond what we normally associate with painting and why the systems
are often called “high-performance” coatings. Let me show you what I mean. These are the standard US federal government
specifications used in department of defense projects. We’re in Division 9, which
is finishes, and if I scroll down, you can see we have a totally different document
for paints and general coatings than the one used for high-performance coatings. There’s
even a more detailed spec used for critical steel structures. If you take a peek into this
specification, you’ll see that a significant portion of the work isn’t the coating application
itself, but the preparation of the steel surface beforehand. It’s estimated that surface prep
makes up around 70% of the cost of a coating system and that 80% of coating failures can be
attributed to inadequate surface preparation. That’s why most coating projects on major
steel structures start with abrasive blasting. The process of shooting abrasive media
through a hose at high pressure, often known as sandblasting, is usually the quickest and most
cost efficient way to clean steel of surface rust, old coatings, dirt, and contaminants, and
cleanliness is essential for good adhesion of the coating. But, abrasive blasting does
more than just clean; It roughens. Most high performance coatings work best on steel that isn’t
perfectly smooth. The roughness, also known as the surface profile, gives the coating additional
surface area for stronger adhesion. In fact, let’s just take a look at a random product
data sheet for a high-performance primer, and you can see right there that the manufacturer
recommends blast cleaning with a profile of 1.5 mils. That means the difference between the
major peaks and valleys along the surface should be around one and half thousandths
of an inch or about 40 microns. It also means we need a way to measure that tiny
distance in the field (in other words, without the help of scanning electron microscopy)
to make sure that the steel is in the right condition for the best performance of the
coating, and there are a few ways to do that. One method uses a stylus with a sharp
point that is drawn across the surface of the steel. The trace can be stored by
a computer and the profile is the distance between the highest peak and lowest valley.
Another option is just to use a depth micrometer with a sharp point that will project into the
valleys to get a measure of the profile. Finally, you can use replica tape that has a layer of
compressible foam. I have an example of several grit blasted surfaces here, and I can apply
a strip of the replica tape. When I burnish the tape against the steel surface, the foam
compresses to form an impression of the peaks and valleys. Here’s what that looks like in a
cross-section view. When the tape is removed, we can measure its new thickness, subtract
the thickness of the plastic liner, and get a measure of the surface profile. Here’s
a look at how the foam looks after burnishing on a relatively smooth surface and a very rough one.
I used my depth micrometer to measure a profile of about 1 mil or 25 microns for the smooth surface
and about 2.5 mil or 63 microns on the rough one. Just to demonstrate the importance of surface
preparation, I’m going to do a little coating of my own here in my garage. I’ve got four samples
of steel here: two I’ve roughened up using a flap disc on a grinder (in lieu of sand blasting), and
two I’ve sanded to a fairly smooth surface. They aren’t mirror surfaces, but the surface profile
is much lower than that of the roughened samples. I also have some oil and I’ll spread a thin coat
on one of the rough samples and one of the smooth ones. I wiped the oil off with a paper towel, but
no soap. So now we have all the phases of youth here: smooth and clean, rough and clean, rough and
oily, and smooth and oily. I’ll coat one side of all four samples using this epoxy product, leaving
the other sides exposed. Notice how the wet paint doesn’t even want to stick to the dirty surfaces,
but it eventually does lay down. I put two coats on each sample, and now it’s into the rustomatic
3000, the silliest machine I’ve ever built. I go into more detail on this in the cathodic
protection video if you want to learn more, but essentially it’s going to dip these samples
in saltwater, let them dry, take a photo, and do it all over again roughly every 5 minutes
to stress test these steel samples. We’ll leave it running for a few weeks and come back to
see how the samples hold up against corrosion. There are countless types of coating systems in
use around the world to protect steel against corrosion. The chemistry and availability of new
and more effective coatings continue to evolve, but there is somewhat of an industry standard
system used in infrastructure projects that consists of three coats. The first coat, called
the primer, is used to adhere strongly to the steel and provide the first layer of protection.
Sometimes the primer coat includes particles of zinc metal. Just like using a zinc anode to
provide cathodic protection, a zinc-rich prime coat can sacrifice itself to protect steel from
corrosion if any moisture gets through. Next the midcoat provides the primary barrier to moisture
and air. Epoxy is a popular choice because it adheres well and lasts a long time. Epoxy often
comes in two parts that you have to mix together, like the product I used on those steel
samples. But, epoxy has a major weakness: UV rays. So, most coating systems use a topcoat
of polyurethane whose main purpose is to protect the epoxy midcoat from being damaged by the rays
of the sun. It’s often clear to visible light, but ultraviolet light is blocked
so it can’t damage the lower coats. The coating manufacturer provides detailed
instructions on how to apply each coating and under what environmental conditions it
can be done. They’ve tested their products diligently and they don’t want to pay
out warranties if something goes wrong, so coating manufacturers go to a lot of trouble to
make sure contractors use each product correctly. They often have to wait for clear or cool days
before coating to make sure each layer meets the specifications for humidity and temperature.
Even the applied thickness of the product can affect a coating’s performance. A coating that
is too thin may not provide enough of a barrier, and one that is too thick may shrink and crack.
Manufacturers often give a minimum and maximum thickness of the coating, both before and
after it dries. Wet film thickness can be measured using one of these little gauges. I
just press it into the wet paint and I can see the highest thickness measurement that
picked up some of the coating. Dry film thickness can also be measured in the field
for quality control using a magnetic probe. Of course, once the coating is applied and dry,
it has to be inspected for coverage. Coatings are particularly vulnerable to damage since they
are so thin, and defects (called holidays) can be hard to spot by eye. Holiday detecting
devices are used by coating inspectors to make sure there are no uncovered areas of steel.
Most of them work just like the game of operation, but with higher voltage and fancier probes.
If any part of the probe touches bare metal, an alarm will sound, notifying the inspector
of even the tiniest pinhole or air bubble in the coating so it can be repaired. Once the
system passes the quality control check, the structure can be put into
service with the confidence that it will be protected from corrosion
for the next several decades to come. Let’s check in on the rustomatic 3000 and
see how the samples did. Surprisingly, you can’t see much difference in the time lapse
view. I let these samples run for about 3 weeks, and the uncoated steel underwent much more
corrosion than the coated area of each square. I also have dried salt deposits all over
my shop now. But, the real difference was visible once the samples were cleaned up. I used
a pressure washer to blast off some of the rust, and this was enough to remove the epoxy coating
on all the samples except the rough and clean one. That sample took a little more effort
to remove the coating. At first glance, the coating appears to have protected all the
samples against this corrosion stress test, but if you look around the edges,
the difference becomes obvious. The rough and clean sample had the least intrusion
of rust getting under the edges of the coating, and you can see that nearly the entire
coated area is just as it was before the test. The smooth and clean sample had much
more rust under the edges of the coating that you can see in these semicircular areas
protruding into the coated area. Similarly, the roughened yet oily sample had those
semicircular intrusions of rust all around the perimeter of the coated area. The smooth and
dirty sample was, as expected, the worst of them all. Lots of corrosion got under the coating on
all sides, including a huge area along nearly the entire bottom of the coated area. It’s not a
laboratory test, but it is a conspicuous example of the importance of surface preparation when
applying a coating for corrosion protection. Like those samples, I’m just scratching the
surface of high performance coating systems in this video. Even within the field of corrosion
engineering, coatings are a major discipline with a large body of knowledge and expertise
spread across engineers, chemists, inspectors, and coatings contractors, all to extend the
lifespan and safety of our infrastructure. We’re back with another attempt of me cooking
dinner while my wife tries to capture that on camera, but this time we have the whole family,
including Wesley the editor. We’ve got two little helpers who, let’s be honest, aren’t that helpful
when it comes to actually cooking the dinner, but they sure do make the process a lot more
fun, especially when Uncle Wesley is in town. HelloFresh is basically a cheat code to a fun
and memorable night in, and you get a delicious meal at the end as a bonus. It’s always fun to get
seasonal recipes and produce straight to our front door. We’ve been using it for years now and have
honestly never had a meal that we didn’t enjoy. And, now that we have picky eaters in the
house, HelloFresh’s Kid-Friendly recipes make it super easy for us to get a dinner on
the table that the big brother is sure to eat. HelloFresh has a bunch of different choices to
help you reach your dietary goals or just try something new including vegetarian,
pescatarian, and fit & wholesome. Give a try at HelloFresh dot com
and use code PRACTICAL65 for 65% off plus free shipping! Supporting our
sponsors helps support the channel. That’s hello fresh dot com and
use code PRACTICAL65. Thank you for watching and let me know what you think.
