How to distinguish natural passivation film from damaged film on 316 stainless steel tubes

How to differentiate between a natural passivation film and a damaged film on 316 stainless steel tubes The core anti-corrosion barrier for fermentation heating loops is a thin, natural chromium-rich passivation film that is formed by 316 stainless steel upon contact with oxygen. This film structure will be destroyed by long-term high-temperature alkali cleansing, chloride erosion, and particle abrasion. In order to prevent weld pitting leakage and organise passivation maintenance in advance, operators require precise distinguishing standards to evaluate the health status of the film during daily patrols. Visual, test, and operational distinguishing indicators are organised in the table below. | Film State | Surface Visual Feature | Ultrasonic & Electrochemical Test Data | Corrosion Resistance Performance | Corresponding Maintenance Advice | | ---- | ---- | ---- | ---- | ---- | | Complete natural passivation film | Uniform silver metallic luster, no discoloration, smooth weld surface | Stable potential value, wall thickness unchanged for multiple inspections | Resist chloride below 50ppm stably | Normal quarterly routine inspection only | | Slightly damaged partial film | Faint light yellow stains concentrated on welds and elbows | Minor potential fluctuation, local thin film detected | Risk of micro-pits under continuous alkali cycles | Arrange offline passivation within 3 months | | Severely damaged fragmented film | Obvious brown rust spots, strip-shaped abrasion marks | Continuous low potential, wall thickness decreasing month by month | Rapid pitting expansion under medium circulation | Stop production for immediate pickling passivation | | Fully lost protective film | Dense rust clusters, exposed dark rough metal matrix | Abnormal low potential, obvious thinning at welds | Through-wall leakage risk within 1–2 production cycles | Replace tube bundle if passivation cannot repair deep pits | The natural passivation film of qualified 316 stainless steel is formed spontaneously when fresh metal contacts air or oxygen-containing cleaning water. It exhibits a homogeneous soft silver gloss without any colour variation, and the weld positions have a consistent smooth texture following the polishing process. Under standard operating conditions, this film is capable of preventing chloride ion infiltration due to its compact and continuous molecular structure. The surface will only retain a uniform metallic colour without blemishes if the film remains intact, even after long-term CIP cycles. The initial indication of the passivation film's degradation is the accumulation of light yellow discolouration on pipeline elbows and welds. These regions are subject to the most severe particle scouring and temperature impact, as the protective film is the narrowest. After partial film decomposition, the yellow substance is loose chromium oxide residue that is incapable of effectively blocking corrosive ions. The most dependable method of differentiation is electrochemical potential testing. Intact natural film maintains consistent high measurements, while damaged areas exhibit a clear downward deviation. Visible brown rust spots will result from severe film damage. During this phase, the chromium-rich barrier has disintegrated in significant portions, allowing the bare metal to directly contact the conductive medium and establish stable corrosion cells. The continuous reduction in thickness at discoloured locations can be captured by ultrasonic wall thickness scanning, which suggests that micro-pits have commenced to form and expand inward. The removal of concealed pits beneath rust stains is not possible through simple surface wiping; only offline pickling passivation can remove contaminants and re-establish a new, entire protective film. Dense rust clusters envelop the tube wall if maintenance is postponed until the film is entirely lost. At this time, the weld has been subjected to deep, irreversible pitting, and the thinned tube wall can only be repaired by slowing corrosion through repeated passivation treatments. The tube bundle is susceptible to abrupt leakage at any moment once this state is achieved, resulting in unanticipated shutdown losses and fermentation medium waste. Production personnel implement a comprehensive evaluation of film quality by integrating quarterly electrochemical potential detection with visual observation during shift patrols for daily on-site management. If yellow discolouration is detected, the inspection cycle should be shortened, and passivation maintenance plans should be developed. To effectively safeguard the natural passivation film of 316 stainless steel tubes and extend the service life of heating equipment, it is essential to strictly regulate the alkali cleaning temperature below 60℃, install front filters to minimise solid particle abrasion, and ensure that there is adequate oxygen flushing after each CIP cycle.