From end to end, our water and wastewater systems are a highly complex combination of collection, conveyance, storage, and treatment structures. In fact, it is hard to emphasize just how critical the continuous and smooth operation of these systems is to a community at large. On the other hand, it is also no surprise to learn how harmful the effects of corrosion can be on these systems over both the short and long term.
While maintaining an aesthetically pleasing finish is one facet to consider within a water and wastewater system, the health and safety of a community—along with the preservation of its natural and financial resources—must be of the highest priority. By gaining a basic understanding of the system's most common corrosion mechanisms, asset owners, design engineers, and other key stakeholders can be better equipped to identify and address the costly and harmful effects of unchecked corrosion.
What is Corrosion?
According to the Association for Materials Protection & Performance (AMPP), corrosion is defined as the "deterioration of a substance because of a reaction with its environment." While that "substance" can include any physical material that can break down, it typically refers to a manufactured or designed material. Within a water or wastewater system, this usually includes substances like non-ferrous metals, ductile iron, and concrete.
When it comes to reducing or eliminating the effects of corrosion, a variety of methods can be employed—from using corrosion-resistant metals and alloys or certain corrosion inhibitors to modifying the design/environment and adding a galvanic or impressed current cathodic protection system. However, the most widely used and cost-effective way to slow corrosion is by applying a protective coating or lining system.
6 Common Mechanisms Behind Corrosion
The idea behind a protective coating is simple: reinforce the substrate with a material that provides inhibitive, galvanic, or barrier protection, thereby slowing or interrupting the corrosion process. When discussing corrosion as it relates to metals, four basic elements exist in what is referred to as a "corrosion cell." This includes the:
- • Anode (most corrosive part of a metal);
- • Cathode (most noble part of a metal);
- • Metallic pathway; and
- • Electrolyte.
If you interrupt or eliminate just one of these elements from the equation, corrosion will be effectively hindered as a result. A coating system that provides any of the above inhibitive, galvanic, or barrier properties can generally accomplish this. However, the corrosion process for concrete is an entirely different story.
Physical forces like impact, abrasion, and erosion are usually the main culprits behind concrete corrosion—meaning that neither inhibitive nor galvanic protection works as effectively as a barrier coating. Additional factors affect the rate at which corrosion occurs within a water and wastewater system, such as hydrogen sulfide and sulfuric acid attack, temperature fluctuations, humidity levels, continued exposure to various process chemicals, environmental chloride, and sulfate and nitrate ions. These factors can sometimes even be the root cause of why this corrosion occurs in the first place.
Below are some of the most common mechanisms of corrosion that you will find within a water and wastewater system:
1. Microbial Corrosion
Microbial corrosion is known by many names, including biogenic corrosion, microbiologically induced/influenced corrosion, and biocorrosion. A corrosion mechanism that can affect both concrete and steel assets, it is a biological process that involves microorganisms associated with the sulfur cycle.
Sulfur-reducing bacteria (SRB) that accumulate in the sludge and sand layers within pipes, manholes, and other structures grow by consuming oxidized sulfur compounds and then excreting hydrogen sulfide (H₂S). When this H₂S reacts with the surface, it can either lower the overall pH level both on the surface and in the structure, or it can be consumed by sulfur-oxidizing bacteria (SOB) to ultimately produce sulfuric acid. This process can be highly destructive to structures that do not have a well-designed coating or lining system that can withstand a low pH environment or sulfuric acid attack. Left unchecked, this can lead to catastrophic structural failure.
2. Galvanic Corrosion
Each metal has a different electrode potential. When two metals come into contact, one metal will preferentially corrode to protect the other. An example of this is when stainless steel and carbon steel are connected. In this case, the carbon steel (or more anodic metal) will sacrifice itself to protect the stainless steel (or more cathodic metal). Typically, this occurs during maintenance or retrofits, when a new pipe, flange, fitting, or bolts consisting of different metals are installed in direct contact with one another.
The rate at which galvanic corrosion occurs will depend on numerous factors, such as the surface area of the anodic vs. cathodic metals, an active electrolyte, and the presence of "salt" ions. This type of corrosion is extremely common and can be eliminated by choosing similar metals, applying a barrier coating to the surface, or simply using an insulative material like Teflon washers to minimize contact.
3. UV Degradation
Everyone loves seeing a bright, shiny new paint job on an elevated water tower. However, the reality is that without the ultra-weatherable topcoats used to protect those structures from the sun's harmful ultraviolet (UV) rays, the actual service life of those tanks would be dramatically reduced.
The epoxy primers and intermediate coats used to provide barrier protection on these structures will readily break down under the presence of UV light. These polyurethane, polysiloxane, and high-performance acrylic resins not only offer an aesthetically pleasing finish but also protection for the coatings underneath.
4. Chloride-Induced Corrosion of Rebar in Concrete
As a substance, concrete possesses excellent compressive strength. However, its tensile strength is relatively poor. For this reason, steel rebar is often used as reinforcement. Concrete is also a highly alkaline substance, with new concrete having a pH of 12-13. That alkalinity helps to provide a passivating layer around the embedded rebar. When chloride ions penetrate the porous concrete substrate and drive through to the rebar, this can quickly break down that passivating layer.
Once the corrosive iron oxide (rust) begins to form, its expansive forces induce mechanical tensile stress within the concrete. This stress, in turn, introduces cracks and spalling of the concrete. Eventually, the rebar itself loses thickness, which can ultimately lead to structural failure. Corrosion in this form can be harder to identify and is often not recognized until it is too late. A protective barrier coating can prevent the ingress of chloride ions throughout the concrete substrate.
5. Concrete Carbonation
Concrete itself is a very porous substance. Given that chloride-induced corrosion typically occurs in a very wet or immersion-type environment, concrete carbonation is generally associated with exposure to environmental carbon dioxide or carbonate ions. Once these substances diffuse into the concrete's porous structure, they begin to react with the concrete, lowering the overall pH level.
As the pH drops, the protective, passivating layer of the steel rebar again begins to break down. The iron oxides that form around the rebar are typically 6-7 times the volume of the concrete matrix. This expansion causes the concrete to crack and break apart yet again as a result. As you can see, it is essential to protect the concrete structure with a barrier coating to prevent this cycle from occurring.
6. Other Physical Forces
Several physical forces can break down both steel and concrete structures within a water and wastewater system. Depending on the structure's location, the freeze/thaw cycle can lift, move and heave a structure, causing significant damage. In a collection system, impact, abrasion, and erosion damage are very common mechanisms behind corrosion.
Whether it be from a municipal jetting cleaner hitting the pipe wall or concrete substrate in a manhole as it attempts to clear a line or from the general daily scouring caused by sand, rocks, and other media, these physical forces can quickly begin to break down even the most thoughtfully designed structures. A fit-for-service protective coating is the easiest way to protect against this type of corrosion.
Whether a structure is being newly designed or a rehabilitation approach is being considered, using a protective coating or lining system is the most cost-effective way to combat the impact of corrosion in a water and wastewater environment. The added upfront costs of new construction are vastly outweighed by the negative costs associated with diminished service life or, even worse, total asset replacement. Protective coatings have preserved vital water and wastewater structures for years, and it's easy to see why they will continue to be relied upon in the same capacity moving forward.