Stainless steel is protected from corrosion by a thin, impervious, invisible surface layer - the passive layer - that consists mainly of chromium oxide. The oxygen content of the atmosphere or aerated aqueous solutions is normally sufficient to create and maintain this passive layer. Unfortunately, surface defects and imperfections introduced during manufacturing operations may drastically disturb this "self-healing" process and reduce resistance to several types of local corrosion. This means that a final cleaning process will often be required to restore an acceptable surface quality with regard to hygiene and corrosion. The extent of and methods for post-manufacture treatment will be determined by the corrosivity of the environment, the corrosion resistance of the steel grade, hygienic requirements (e.g. in the pharmaceutical and food industries) or by purely aesthetic considerations. Consideration must also be paid to local environmental requirements. Both chemical and mechanical cleaning methods are available. Good design, planning and methods of manufacture can reduce the need for finishing work and thus reduce costs. The influence of defects, and ultimately their removal, must be considered when manufacturing to specifications that relate to certain surface quality requirements.