Inside a synthetic fibre and plastics factory, a 600-litre leak of raw acrilonitrile (ACN) containing 10% hydrocyanic acid (HCN) spilled from a black steel (black PVD) tank located in a retention basin and fitted with a stainless steel extension and buttering. This leak was caused by an internal weld break on the connection of the 2 stainless steel tank parts. The tank was initially modified in 1974, following an hydrogen cyanide (HCN) leak due to stress corrosion of the black steel. As a result of this break, the acid attacked the (outer wall) of the black steel tank. Given that the base of the tank had an opening, the incident was quickly detected. The factory operator responded by installing a foam film in such a way that the acid was prevented from evaporating. Tank contents were immediately transferred to another storage facility and the defective tank was isolated. The ACN spread over the retention basin floor was channelled to the site’s waste treatment centre. The investigation undertaken revealed that stress corrosion might have initiated the incident. The operator explained that that this weld had been performed with cost savings as a major concern. After the incident, the operator replaced the tank by a fully stainless steel container. A second storage unit of the same type was also scheduled for replacement.