Acorrosion inhibitor oranti-corrosive is achemical compound added to a liquid or gas to decrease thecorrosion rate of ametal that comes into contact with the fluid.[1] The effectiveness of a corrosion inhibitor depends on fluid composition anddynamics. Corrosion inhibitors are common in industry, and also found in over-the-counter products, typically in spray form in combination with alubricant and sometimes apenetrating oil. They may be added to water to prevent leaching of lead or copper from pipes.[2]
A common mechanism for inhibiting corrosion involves formation of a coating, often apassivation layer, which prevents access of the corrosive substance to the metal. Permanent treatments such aschrome plating are not generally considered inhibitors, however: corrosion inhibitors are additives to the fluids that surround the metal or related object.
The nature of the corrosive inhibitor depends on (i) the material being protected, which are most commonly metal objects, and (ii) on the corrosive agent(s) to be neutralized. The corrosive agents are generally oxygen, hydrogen sulfide, andcarbon dioxide. Oxygen is generally removed by reductive inhibitors such asamines andhydrazines:
In this example,hydrazine converts oxygen, a common corrosive agent, to water, which is generally benign. Related inhibitors of oxygen corrosion arehexamine,phenylenediamine, anddimethylethanolamine, and their derivatives.Antioxidants such as sulfite andascorbic acid are sometimes used. Some corrosion inhibitors form apassivating coating on the surface bychemisorption.Benzotriazole is one such species used to protectcopper. Forlubrication,zinc dithiophosphates are common - they deposit sulfide on surfaces.
The suitability of any given chemical for a task in hand depends on many factors, including theiroperating temperature.[3]
Corrosion inhibitors are commonly added to:
Corrosion of tap water pipes can be influenced by a number of factors such as the pH, buffering capacity, and hardness.[6] Methods of control include directly adjusting the pH, addingphosphates,silicates as an alternative corrosion inhibitor, or addingbicarbonates for buffer.[2]
Orthophosphates may be added intap watertreatment systems to prevent leaching of lead and copper from water pipes and reduce the ion content in tap water to safer, legal levels.[2]Polyphosphates can be used to control iron and manganese, which cause discoloration, but do not control lead and copper. The water industry commonly uses a blended-phosphates formulation to deal with both issues.[7] Phosphates convert any leached ions into a layer ofscale that acts to separate the metal piping from the water.[8]
Phosphate-type inhibitors may causeeutrophication issues downstream or directly encourage algal growth in uncovered, treated water reservoirs. As a result, local water systems may elect to use alternative methods.[9]
In areas with widespread lead and copper piping systems, corrosion control using inhibitors and monitoring techniques is central to water safety. Figures such as the chloride-to-sulfate mass ratio (CSMR) can be used to estimate the risk ofcorrosion at galvanic connections (i.e. desimilar pipe/solder connections, such as a lead-to-iron transition). The 2014Flint water crisis was caused by a combination of source water change and a lack of corrosion control. The new, higher-CSMR water not only dissolved lead and iron from the pipes themselves, but also broke up previous layers of lead-containing rusty scale in pipes, allowing them to enter the water supply.[8]