Damage such as micro cracks, layer delaminations, corrosion or barely visible impact damage (BVID) could irreparably affect the integrity of the structure. These defects are not ever detectable by the common inspection techniques based on the ultrasonic wave propagation. However, a number of techniques based on nonlinear wave behaviour have been recently developed to improve the sensitivity of ultrasonic methods. The nonlinear acoustic approach proposed in this work relied on generation of new frequency generation due to defects. The spectral changes are caused by nonlinear local dynamics of defects of various scale and nature due to contact between crack surfaces. A standard Air Coupled ultrasound (ACU) system arranged with 88 transmitting elements and 1 receiving element focused on the same point (N=88 mm) with a central frequency (f0) of 41 KHz was used to excite corroded samples. Results showed that the intact parts of the material outside the defect vibrate linearly, i.e. with no greatly frequency variation in the output spectrum, whilst a small cracked defect behaves as an active radiation source of a new frequency component (2f0). For the nonlinear ultrasonic testing, the second order nonlinear parameter (β) was chosen as the nonlinear feature to damage identification. In conclusion this research work demonstrated that nonlinear techniques are suitable for numerous classes of defects, such as fatigue cracks and corrosion (micro-cracks).