『Original by HaiChuan Translation Team』Introduction to Corrosion Inhibitors (Part 1)

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English title: An Introduction To Corrosion Inhibitors. Original article link: Click here. Original author: ankur2061. Translator ID: @kid_ptd. Proofreader ID: @MyChemicalRomance. Currently, there are already quite a number of theories and ideas focused on the research of corrosion inhibitors for suppressing corrosion on the “Cheresource forum”. This article mainly introduces some knowledge related to \"corrosion inhibitors\". The following sections will provide a detailed explanation, and the references related to the main content of this article are listed at the end. There are many diverse ways to use corrosion inhibitors to reduce the corrosion rate of a system. In the crude oil extraction and refining industry, corrosion inhibitors are typically used as the first line of defense against corrosion. Currently, there are numerous research projects on corrosion inhibitors. However, most of the known information still comes from laboratory and field tests as well as error correction. The laws, equations, and theories that can be used to guide the use and development of corrosion inhibitors remain very limited. By definition, corrosion inhibitors are a general term for certain types of chemicals; when added to the application environment in low concentrations, they can effectively reduce the corrosion rate. The effectiveness of a corrosion inhibitor can be expressed using the following equation: Corrosion inhibitor effectiveness (%) = (CR without inhibitor – CR with inhibitor) / CR without inhibitor. In this formula, CR without inhibitor refers to the corrosion rate in a system where no corrosion inhibitor is used ； CR refers to the corrosion rate with corrosion inhibition, that is, when a corrosion inhibitor system is used. Generally, the efficiency of corrosion inhibitors increases as their concentration rises (for example, a typical corrosion inhibitor provides 95% corrosion inhibition efficiency at a concentration of 0.008%, while at a concentration of 0.004%, the efficiency is only 90%). Synergistic effects often arise between different corrosion inhibitors and a controlled system environment; therefore, mixtures are commonly used in commercial formulations. Scientific papers and technical articles on corrosion have listed many substances with corrosion-inhibiting properties, along with relevant descriptions. But among these substances, only a very small portion is used in practice. To some extent, this is because the requirements for the performance of corrosion inhibitors often go beyond the scope of mere metal protection. It is also very important to examine its cost, toxicity, practicality, and environmental friendliness. To protect metal elements from corrosion in certain typical corrosive environments, there are several corrosion inhibitors that have been successfully used in industry, as listed in the table below. Commercial corrosion inhibitors come in various trade names or labels, but their chemical composition is often little disclosed, or even completely unknown. Sometimes it can be very difficult to simply distinguish products from different sources, as they may contain the same basic corrosion-resistant ingredients. Formulas for commercial products usually consist of one or more corrosion inhibitors, along with other additives such as surfactants, film-forming enhancers, demulsifiers, deoxidizers, and so on. Given the different dissolution and dispersion properties, as well as the application and performance characteristics of the products, the packaging used for corrosion inhibitor solutions is extremely important. Table 1: Some corrosive systems and the corresponding corrosion inhibitors
System | Corrosion Inhibitor | Metal Concentration
Acids | Ethylaniline hydrochloride | Iron: 0.5%
| Thiobenzothiazole | Iron: 1%
| Pyridine + Phenylhydrazine | Iron: 0.5% + 0.5%
| Rosin amine + Ethylene oxide | Iron: 0.2%
| Phenylacridine sulfate | Iron: 0.5%
| Sodium iodophosphate | Iron: 200 ppm
Others | Thiourea | Iron: 1%
| Sulfonated castor oil | Iron: 0.5–1.0%
| Arsenic trioxide | Iron: 0.5%
| Sodium arsenate | Iron: 0.5%
Water – Potable water | Calcium bicarbonate | Steel, cast iron: 10 ppm
| Polyphosphates | Iron, zinc, copper, aluminum: 5–10 ppm
| Calcium hydroxide | Iron, zinc, copper: 10 ppm
| Sodium silicate | Iron, zinc, copper: 10–20 ppm
Cooling water | Calcium bicarbonate | Steel, cast iron: 10 ppm
| Sodium chromate | Iron, zinc, copper: 0.1%
| Sodium nitrite | Iron: 0.05%
| Sodium dihydrogen phosphate | Iron: 1%
| Morpholine | Iron: 0.2%
Boiler water | Sodium dihydrogen phosphate | Iron, zinc, copper: 10 ppm
| Polyphosphates | Iron, zinc, copper: 10 ppm
| Morpholine | Concentration variable
| Hydrazine | Iron deoxidizer
| Ammonia | Iron neutralizer
| Stearamine | Iron concentration variable
Engine coolant | Sodium chromate | Iron, lead, copper, zinc: 0.1–1%
| Sodium nitrite | Iron: 0.1–1%
| Sodium tetraborate | Iron: 1%
| Ethylene glycol/water | Sodium tetraborate + Thiobenzothiazole | All metals: 1% + 0.1%
Oilfield brine | Sodium silicate | Iron: 0.01%
Quaternary ammonium compounds | Iron: 10–25 ppm
| Imidazolines | Iron: 10–25 ppm
Seawater | Sodium silicate | Zinc: 10 ppm
| Sodium nitrite | Iron: 0.5%
Calcium bicarbonate | All metals; pH-dependent
| Sodium dihydrogen phosphate + Sodium nitrite | Iron: 10 ppm + 0.5%
In the next issue, we will introduce specific types of corrosion inhibitors as well as several corrosion inhibition methods. Stay tuned!