Abrahan Puente, Engineering Manager, AUGE Industrial, www.auge.com.mx
It is no surprise that a common goal among the wind industry’s participants is to lower the total cost of ownership associated with wind farms while not sacrificing on health, safety, and environmental issues.
To help reach the goal, this article focuses on the significance of removing entrapped hydrogen from bolt steel and how an innovative approach minimizes the risk of hydrogen embrittlement without cutting back on quality or safety.
Seismic regions around the world require hardened high-tensile strength anchoring systems. These must have a minimum 1,040 MPa tensile strength (Reference ISO 898-1).
Making such materials comply with this stress value requires hardness usually above 35 HRc, because these are thermally treated by means of quenching and tempering. Wind-turbine OEMs require hot-dip galvanization to avoid the natural phenomenon of corrosion.
Hot-dip galvanizing typically consists of three steps per the American Galvanizers Association, including surface preparation, galvanizing, and inspection. During surface preparation, the steel is dipped in acidic solutions such as sulfuric acid or hydrochloric acid as a way to remove surface impurities and oxides.
The problem arises when coating a high-strength hardened alloy by hot-dip galvanizing. During this first step, there is a high probability of a phenomenon called “Environmental Hydrogen Embrittlement” occurring because the material contacts the acid medium.
Here’s what goes on at an atomic level: Hydrogen embrittlement results when metals literally absorb hydrogen. It is the smallest molecule in nature. When it diffuses along the grain boundaries, hydrogen atoms are absorbed into the metal lattice and diffused through the grains, tending to gather at inclusions or other lattice defects.
Disassociated hydrogen ions take little space, but when hydrogen ions combine to form hydrogen molecules they take up tens of thousands of times more space.
This applies stress on a granular level and may form cracks, thus causing the part to fail when additional outside stress is applied during use. Also, this usually results in a loss of ductility or load carrying capacity, which may cause catastrophic brittle failures at applied stresses well below the yield strength. Failures occurring in service are serious and may be costly.
Hydrogen must be in the atomic form to impose damage to steel. Because hydrogen has the smallest atomic mass, it can enter the molecular structure of the steel. This is not true when two hydrogen atoms combine to form a stable H2 molecule. Hydrogen in molecular form is too dense to penetrate a steel structure.
Because hydrogen is exceptionally mobile, it quickly penetrates any recently formed cracks, lesions, or material surface discontinuities, and creates high stress areas within the steel structure. When embrittlement failures occur, they often significantly increase costs and lead times associated with the development of a project.
To prevent this phenomenon, it is common to use alternative methods of cleaning such as sand blasting or air blasting instead of using acid solutions. Also, it may be recommended to use post-baking for dehydrogenation.
To ensure risk reduction, our company has developed AUGE Rhino, an alternative cleaning method consisting of an alkaline de-scaling process. The results are in full compliance with ASTM A153 and EN 10684 (thickness, adhesion test, etc.). Upon completion of laboratory testing and customer approval, the company applies hot-dip galvanized anchoring systems safely, through a process free of acidic solutions. This process allows reducing cost and lead-time by reducing the need for baking the steel.
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