1. HYDROGEN EMBRITTLEMENT : CAUSES, EFFECTS & PREVENTION ABHISHEK KUMAR BT/ME/1601/002

2. CONTENTS Introduction Causes Mechanism Effects Prevention Techniques

3. INTRODUCTION Embrittlement is a loss of ductility of a material, or making it brittle. If embrittlement occur,due to the effect of hydrogen absorption then it known as Hydrogen Embrittlement. It is the process by which metals such as steel becomes brittle and fracture due to the introduction and subsequent diffusion of hydrogen into metals. This is the result of accidental introduction of hydrogen during forming and finishing process. This issue is caused by material properties (diffusion of hydrogen), environment, and stress.

4. Hydrogen may be introduced during : [1] During Melting & Entrapped during Solidification, [2] Anodic Reaction during Corrosion, [3] Hydrogen Gas Welding & Moistured Electrode

5. The Chief characteristics of Hydrogen Embrittlement : [1] Strain Rate Sensitivity increases, [2] Susceptibility to Delayed Fracture increases. Hydrogen Embrittlement is enhanced by slow strain rates. At low temperatures and high temperatures hydrogen embrittlement is negligible, but it is most severe at Room Temperature for example steel. Slow bend test and Notched and Unnotched tension tests will detect hydrogen Embrittlement by a drastic decrease in ductility, but notched–impact tests are of no use for detecting the phenomenon.

6. MECHANISM The exact mechanism of hydrogen embrittlement is not well known. The initial causes is the same: penetration of atomic hydrogen into the metal structure. Most of the mechanisms that have been proposed for hydrogen embrittlement are based on slip interference by dissolved hydrogen. This slip interference may be due to accumulation of hydrogen near dislocation sites or microvoids, but the precise mechanism is still in doubt.

7. PROPOSED MECHANISM Hydride-Induced Embrittlement, HIE (Second-phase mechanism) Hydrogen-Enhanced Decohesion Mechanism, HEDE (brittle fracture) Hydrogen Enhanced Localized Plasticity-Mechanism, HELP (ductile fracture)

17. PREVENTION TECHNIQUES Reducing Corrosion Rate Reducing Corrosion Rate Hydrogen embrittlement occurs frequently during pickling operations where corrosion of the base metal produces vigorous hydrogen evolution. By careful inhibitor additions, base-metal corrosion can largely be eliminated during pickling with a susequent decrease in hydrogen pickup.

18. PREVENTION TECHNIQUES Clean Steel Using Clean Steel Rimmed steels tend to have numerous voids, and the subtitution of killed steel greatly increases the resistance to hydrogen interstitials for embrittlement because of the Less number of voids in this material.

19. PREVENTION TECHNIQUES Baking Baking Hydrogen embrittlement is an almost reversible process, especially in steels. That is, if the hydrogen is removed, the mechanical properties of the treated material are only slightly different from those of hydogen-free steel. A common way of removing hydrogen in steels is by baking at relatively low temperatures at 200-300 F.

21. PREVENTION TECHNIQUES Proper Welding Practicing Proper Welding Low-hydrogen welding rods should be specified for welding if hydrogen embrittlement is a problem. Also, it is important to maintain dry conditions during welding since water and water vapor are major sources of hydrogen.

22. PREVENTION TECHNIQUES Substituting Alloys Substituting Alloys The materials most susceptible to hydrogen embrittlement are the very high-strength steels. Alloying with Ni or Mo reduces susceptibility. Because, Nickel-containing steels and Nickel-base alloys have very low hydrogen diffusion rates and best way to prevent from hydrogen embrittlement.

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