Oct-2020

What is hydrogen embrittlement in metals, where in the plant is it most likely to occur, and how can we avoid it? (PTQ Q&A)

Responses to a question in the Q3 2020 issue of PTQ

Various from Kurita Europe and SUEZ Water Technologies & Solutions

Article Summary

Berthold Otzisk, Senior Product Manager, Process Chemicals, Kurita Europe - berthold.otzisk@kurita-water.com
Hydrogen embrittlement is a kind of stress corrosion cracking, resulting in several forms of damage. Known effects are loss of ductility and fracture strength, and macroscopic damage due to entrapment at mechanical interfaces. The reduction of hydrogen is the corresponding cathodic reaction to the anodic reaction to initiate aqueous corrosion. Cathodic hydrogen adsorbs on the metal surface, while gaseous hydrogen adsorbs in the molecular form. Nascent hydrogen is a chemisorbed species on the metal surface, which can enter the metal. Gaseous hydrogen must first dissociate to form atomic hydrogen. The crystal structure of iron based alloys has small holes between the metal atoms. Between these holes there are wide channels. The hydrogen has a low solubility in such alloys, but a relatively high diffusion coefficient.

In aqueous systems, the entry of hydrogen is promoted by poisons that inhibit the recombination of nascent hydrogen on the metal surface. Hydrogen sulphide, other ionic species of sulphur, antimony, phosphorus, bismuth, and cyanides are such poisons. Hydrogen embrittlement is observed at plants where those impurities are present. Hydrogen embrittlement is strongly influenced by the strength level of the metal, where hydrogen sulphide is known to be most aggressive in promoting hydrogen entry. Common metals and alloys are qualified to heat treatment and to strength level in terms of the resistance to hydrogen induced failures. These qualifications are reported in NACE standards RP-04-75, MR-01-75, and MR-01-76. The selection of suitable alloys can help to reduce the risk of hydrogen embrittlement.

Please download the published PTQ article (Q2 2009, Hydrogen-induced cracking and blistering) where a very effective corrosion inhibition is described. Powerful filming amines can help to reduce the risk of hydrogen embrittlement by providing a reaction barrier for the nascent hydrogen.

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Collin Cross, Senior Product Analytics/Support Manager, SUEZ Water Technologies & Solutions, collin.cross@suez.com
Hydrogen embrittlement (HE) is a type of damage suffered in various high strength steels. It is caused by penetration of monoatomic hydrogen into the metal. It then recombines to form molecular hydrogen leading to internal pressures that weaken the intergranular structure. Various forms of specific damage occur from HE, but generally all are versions of cracking. The differences in types of cracking are due to the specifics surrounding impact on ductility, types of environments, and types of stresses leading to the damage. The alloys most affected by this mechanism are certain carbon alloy steels, certain stainless steels, and some high strength nickel alloys.

Units affected by HE in refineries are units that contain environments with high concentrations of hot hydrogen, proper chemical conditions, the right type of steel, and are subjected to various types of mechanical stress. There are also several special reasons HE can occur, such as welding practices, cleaning practices, and metal manufacturing processes. While these are important mechanisms that can cause HE, for the discussion here we will focus on unit types that provide the correct hydrogen-rich chemical environments and which are often the most at risk.

For HE to occur, generally three factors are necessary, including high concentrations of hot (<300°F) gaseous hydrogen, alkaline conditions, and poisoning agents that slow the recombination of monoatomic hydrogen as molecular hydrogen. Examples of common poisoning agents are cyanide, arsenic, and sulphides. The name for the type of corrosion that favours these conditions is ‘wet H2S corrosion’; it most frequently occurs in cracking units such as the FCC unit, hydroprocessing units, and cokers. However, units that take feeds from cracking units such as amine units, sour water service units, and HF alkylation units can also experience wet H2S corrosion.

To avoid HE, many strategies should be employed. Routine inspection and monitoring help the detection and mitigation of HE. Proper alloy, post weld heat treating (PWHT) of components, proper welding practices, and proper start-up/shutdown procedures of at-risk units are all important. Protective linings can also be used in the proper circumstances to prevent hydrogen reactions from occurring as favourably. Finally, chemical mitigation can also be used to control HE to a large extent.

Chemical treatment generally falls into two categories: scavenging and passivating. Traditionally, the use of a scavenger was called for, and many equipment OEMs still call for this method of mitigation. The most common scavengers used are either ammonium or sodium polysulphide. These chemicals are often called ‘cyanide scavengers’ because they work to destroy the poisoning agent and thereby lower the concentration of monoatomic hydrogen to prevent its penetration. While polysulphide scavengers work well and are still used today, they have several negative side effects that have caused a decline in usage in recent decades considering the development of newer and less problematic chemical methods. The problematical side effects of polysulphides include downstream equipment fouling, toxicity, pumpability, and handleability.

Newer chemical mitigation methods surround the use of specialised high pH passivating inhibitors, or filmers, somewhat like those commonly used in other fractionator overhead corrosion service. While filmers do not directly eliminate the cyanide (or other poisoning agents) as do polysulphides, they do help to prevent monoatomic hydrogen penetration and subsequent HE. HE is prevented in this case because the passivating film fosters rapid recombination from monoatomic hydrogen back to molecular hydrogen outside the metal, thus preventing the penetration necessary for embrittlement to occur. Many refineries today prefer the use of filmers to polysulphides due to their lack of negative side effects, favourable economics, and strong ability to prevent HE.