This is fourth in a series on corrosion control; find the previous posts here.
Within the world of corrosion control, coatings can include a number of different materials and processes, from paints to plating, to enamel and even bio films. Functionally, however, corrosion control coatings are not so different; they mainly serve one or both of two purposes: physical barrier and sacrificial anode. In order to function as a physical barrier, a material must simply prevent the chemicals which cause corrosion, i.e. water, and oxidizers like oxygen, chlorides or sulfur compounds, from touching the metal being protected. We'll see in a moment how difficult this can be. Most paints and spray coatings serve only the physical barrier function and therefore do not satisfy protection needs in extreme environments. That's where the sacrificial anode function needs to be involved. To act like a sacrificial anode, a coating needs to use the properties of galvanic corrosion and the galvanic series to introduce another metal or compound which will react with oxidizing chemicals with greater ease, sacrificing itself so that the protected metal remains untouched. This is also known as cathodic protection. This function is most commonly seen as metal plated with zinc, which acts as a sacrificial anode.
Materials that work only as a physical barrier against corrosion face an insurmountable challenge. The two most important properties for a coating to act as a physical barrier are permeability and adhesion. Water vapor permeation of coatings is inevitable, which means physical barrier coatings only differ in the amount of time the process takes. Coatings, like paints or sprays, which include lamellar pigment particles, like leafing aluminum or micaceous iron oxide, tend to protect better than others because the pigment particles align, making permeation more difficult for water vapor. Tight adhesion to the protected metal also affects the rate of vapor permeation; coatings with lower viscosity can fill all of the nooks on rough-surface metal, avoiding gaps between the coating and metal which can be cradles of corrosion. The process of application of a paint or spray protection has an impact on both the permeability and the adhesion properties of a coating. Thicker coatings protect better, especially when done as several layers, and carelessly applied coatings with holes or places with poor adhesion perform similar to no coating at all. Once corrosion finds an in-road, no matter how small, it will destroy everything in its path, which means full coverage of the coating is paramount. Of course there are environments in which physical barrier coatings simply fail too quickly. These extreme conditions require some kind of sacrificial anode protection.
The word "extreme" in that last sentence can no longer be replaced with "unusual", as sulfur levels in East Asia are continuing to rise, affecting the corrosion characteristics of air throughout the Pacific and, indeed, the entire globe, or at least the whole Northern Hemisphere. A consequence of this is the increased need for sacrificial coatings. Sacrificial coatings come in many forms — there are paints and sprays with all kinds of phosphates, chromates and other reactive chemicals — but by far the most common is the galvanization of metal with zinc, also known as plating, via either electrogalvanization or the more robust method of molten hot dip. These processes adhere a layer of zinc to the metal which acts as both a physical barrier and a sacrificial anode, making full coverage less of a necessity, though still a goal.
Even with the best application and ideal properties, sometimes materials alone are simply not enough, and systems need to be brought in to control corrosion with finer precision. Impressed current corrosion protection systems take over when conditions are too extreme for coatings to function. Learn more about Impressed Current Cathodic Protection in this post. Even ICCP systems have their limits, though. The unstoppable force of corrosion always wins.