This post was spurred on by work that I did illustrating hundreds of pages for our Construction Packet.
As an illustrator, I spend extra time nailing down details, just to eliminate potential sources of confusion. While looking into salt weathering, I came across some really interesting science. There was so much depth to the information, I had a hard time pulling away from the section. You could probably fill another 200 page packet just with little drawings of various materials and how they react to salts.
Many of the articles I referenced were general construction, or engineering papers. As such, they were concerned with all kinds of salts. Initially, I thought that our friend MgSO4 (magnesium sulfate, or epsom salt) was one of the most damaging salt crystals out there. I’d seen what it could do to Float On, and the long pointy crystals from epsom salt just look mean, don’t they? The fact of the matter is, though, there are other salts that are are much worse. NaCl (table salt) is an incredibly corrosive salt to metals, and Na2SO4 (present in sea, rain and groundwater) takes the cake for concrete damage.
This is not to say that MgSO4 won’t do its best to destroy everything in its path. We’re not just dealing with a little bit of salt, as is found in a solute in seawater and rain; we’ve got gallons and gallons of a solution so saturated that even small changes of temperature can leave trails of crystals sparkling all over our tank walls. When I say there are worse salts, I’m actually saying, “Yeah, so the entire army of Mordor is here, but hey, at least the Uruk Hai aren’t coming.”
The armies of MgSO4 have many ways to attack our shops. Enjoyably enough, “Attack,” as used here, is actually a technical term – engineers call the process by which the mechanical forces of crystals break apart materials a “physical salt attack.”
What makes this phenomenon so pervasive is how salt dissolves and reforms in the appearance or absence of water. When it dissolves, the salt particles are smaller. Many salts are hydrophilic, so they bond quickly with water and get everywhere that water gets. The water can do something that the salt can’t though: evaporate. When it does that, the salt particles expand as they change states to a solid. If anything is in its path when this happens, it will get broken and pushed aside. When you understand this, “attack” seems like an ever more appropriate term for process.
The amount of damage done in a physical salt attack is affected by many variables including: the saturation of salt solution, the temperature, the humidity of the air, how porous the material is, and the shape of the pores in the material.
Collection and Crystallization Attacks
The first step towards a physical salt attack is the collection of salt solution. If the surface on which the salt solution collects is not porous, the physical salt attack has already ended. The crystals form out in the open, growing as they please, doing no damage (at least mechanically). The formation of crystals on the surface of a material is called “efflorescence”. If that material is porous, like concrete, wood, or even cracked paints, that salt is likely to trap itself beneath the surface. This is called “sub-efflorescence” or “subflorescence.”
You can imagine a crowd of salt particles trying to escape a room as water evaporates. In materials where the opening to a pore is roughly as big as the pore itself (like a hallway) the damage will be lessened. If that pore is smaller at the exit, crystals will build up as the exit narrows (like everyone trying to leave a crowded room, all at once, through a single door). The more severe the bottleneck the stronger the force of the crystal build-up on the walls of the pore. Concrete is a good example of terrible bottlenecks. With many different sizes of particles, the pores are variable in size. This means that as the water evaporates and the salt expands, it also burrows deeper into the material, basically ripping away layers of concrete with it.
Crystals form when a salt solution becomes “supersaturated.” With our solution already at saturation point, it only takes a small amount of evaporation, or a small drop in temperature to start crystallization. The faster a crystal forms in a confined space, the more damage it will do. You might be thinking, “Well I’ve got these awesome climate controlled rooms, and super humid air to prevent that from happening! No problem, right?” No – in our warm, humid environments, if we don’t clear up puddles on porous materials quick, we’ve got even worse damage coming.
A fast drying solution can superficially tear things up, but a slow drying solution can irreparably compromise the integrity of a material. It’s bad news either way.
As I mentioned earlier, epsom salt is hydrophilic (it loves water), and the further into the material the water goes, the more salt gets pulled in with it. The longer a salt solution sits on a porous surface, the more it’s able to embed itself. Water damage on its own is no joke, but then the material dries and salt crystals expand and really tear things up. Dried salt crystals can even pull more water from the air. This small amount of moisture will dissolve small crystals, but not larger ones. The large crystals can then attract the newly created salt solution, and grow even further.
Not even non-porous surfaces are safe.
Salts (both sulfates like epsom salt and chlorides like table salt) can react chemically with many minerals found in masonry and cement. I admit that I am not a chemist, and my understanding of these complex reactions is shaky. The role this plays in salt weathering is also something that is currently still being studied.
In any case, it’s interesting enough to note that it’s possible for epsom salt to dissolve cement and other stonework without a single crystal forming. You can recognize a sulfate attack by crumbling or spalling, and white deposits. (You see this a lot on brickwork outside, or on sidewalks in areas where salt is used to melt ice in the winter.)
MgSO4 also aids oxidation of metals. You probably recall that metals tend to rust in the presence of water and oxygen. Typically this process starts with a small amount of dissolved metal in the water (let’s say iron, because that’s one that’s well-described for non-chemists like me). The dissolved iron ionizes the water – that is, it puts out some free electrons. Hydrogen and oxygen are buddies in water, because oxygen has electrons that hydrogen wants. If you offer hydrogen a different electron, it’ll take that, and just run off. This leaves a “hydroxide” molecule all by it’s lonely self. Hydroxide settles itself in with the various iron atoms that are hanging around, making iron oxide, iron hydroxide and other combinations of iron, hydrogen and oxygen. (The different colors of rust are caused by the chemical makeup of the rust).
MgSO4 ionizes water, kicking off this whole process with more hydroxide atoms and more gusto.
Ionized water is more conductive, and can incite an interesting effect. “Galvanic corrosion” occurs when two different kinds of metals touch in a conductive solution. This makes a battery. One of the metals will oxidize like crazy, the other will generally be untouched. I imagine this is a much more common problem for ships and shipyards where metal is more widespread than for float centers, where aesthetics and cost generally limit the use of metal.
Some metals are able to resist corrosion. Common red rust (iron oxide) flakes away easily, revealing more iron atoms for hydroxide to buddy up to. Under the right conditions, black rust can form. It sounds scary, but black rust (Fe2O4, or magnetite) is much more adherent to its host metal. Magnetite (with that lovely buildup of oils) is part of what keeps old cast iron from corroding away even after decades of use.
Aluminum is often used as a cheap, highly corrosion resistant metal. You see it not only to hold hot-dogs, but also in public pools. Aluminum coats itself almost instantaneously with aluminium oxide in the presence of oxygen. Aluminum oxide is highly adhesive, and it’s quick formation makes corrosion unlikely, even after physical damage like scratching.
Stainless steel is an alloy that includes chrome, which oxidizes to the durable chromium oxide. Chromium oxide likes to stick to its stainless steel, but doesn’t like other things sticking to it. As a result, float centers, shipyards, and pools all use stainless steel in their designs. Other alloys are sometimes included to improve resistance to particular chemical compounds. Float tank centers use high grade stainless steel for its extreme resistance to salt.
Stainless steal is still susceptible to corrosion under specific circumstances. Chromium oxide forms slightly slower than aluminum oxide, which makes it slightly more at risk from physical damage. Scrubbing stainless steel with iron, in particular, can leave iron flakes on the surface, and kick off the oxidation process. Another weakness occurs when chlorides (such as table salt) are mixed with water; they have the possibility to react and create hydrochloric acid, which destroys the chromium oxide barrier.
I feel like I’m only scraping the efflorescent surface of this salt thing. Luckily,you don’t need to know exactly how rust works or the precise formula for the perfect crystal formation to know how to keep your float tank rooms from falling apart.
These details are fascinating and can be helpful in understanding how your (seemingly) bulletproof floors, walls, and tanks get destroyed, but the takeaway is surprisingly simple. It comes down to this: use non porous surfaces, and keep them clean. Don’t let Mordor gather its forces, because it will bring down some of the toughest of walls.
For more detailed information on building materials and how to keep the salt armies at bay, check out our blog, our podcast, and our expansive Construction Packet… or shoot us a message and ask a question.