Hydrogen can weaken steel without visibly corroding it
Steel usually tolerates some stretching and bending before it breaks. That ability to deform is called ductility. Hydrogen can reduce it: individual hydrogen atoms are small enough to enter the steel’s crystal structure, where iron atoms are arranged in a repeating pattern. The metal may still look intact, but it becomes more vulnerable to cracking under load.
Stress gives tiny cracks a place to grow
A bridge tendon is a high-strength steel element that is stretched tight to help hold concrete in compression. That tension is useful for supporting the bridge, but it also keeps the steel under sustained stress. Near a microscopic flaw or other highly stressed spot, hydrogen can make it easier for a crack to begin or advance. Instead of bending and redistributing the load, the steel may fracture with little warning.
This is known as hydrogen embrittlement. It is not the same as rust: rust gradually consumes metal at its surface, while hydrogen embrittlement can undermine a stressed component from within. The exact path by which hydrogen assists cracking depends on the steel and conditions, so the important point is the combined effect of hydrogen, stress, and a susceptible material.
Why the bridge can sag later
A tendon can continue carrying force even after damage has begun. But as cracks grow, its remaining steel has less capacity to hold the concrete in compression. If enough support is lost, the concrete can deform under the bridge’s weight and the structure may sag. The visible change can therefore arrive after a period in which the weakening was difficult to see: the bridge’s support was deteriorating inside its steel, not necessarily announcing itself with obvious surface damage.
