Detecting corrosive sulphur in a transformer oil is the start of a problem, not the end of it. Once an oil is found to form copper sulphide, an asset manager has to decide what to do, how urgently, and whether the treatment will still be working in a decade. CIGRE Technical Brochure 625 turns a positive test into a defensible decision. Produced as the successor to the earlier brochure that first mapped the failure mode, it consolidates global service experience into a practical framework for risk assessment and long-term mitigation.
What it covers
The brochure works through the full arc of the problem. It sets out how copper sulphide forms — the parallel processes of attack on bare copper and migration into the cellulose, and the factors that drive them, such as temperature, oxygen, base-oil composition, and the reactivity of the sulphur species present. It then assembles failure cases and inspection findings from a worldwide survey to identify which transformers are genuinely at risk and which carry corrosive oil without ever failing.
From there it turns to mitigation, evaluating the three principal field options — adding a metal passivator, changing the oil, and removing corrosive sulphur from oil that stays in service through reclamation or chemical treatment — and examining their long-term effectiveness and side effects rather than their immediate result. The work closes with monitoring guidance and a consolidated decision flow that ties detection, risk factors, and mitigation choice together.
The scope is framed around mineral oil-filled equipment: power transformers, reactors, auto-transformers, and generator step-up units. It is a guidance brochure rather than a routine oil-quality test standard.
Why it matters in practice
The central insight is that a corrosive oil is not automatically a failing transformer. Risk depends on the combination of corrosivity, operating temperature, design, and time — which is why the brochure frames the question as risk assessment rather than pass/fail, preventing both complacency and unnecessary, costly intervention.
It is equally candid about mitigation trade-offs. Passivation suppresses the reaction but is consumed over time and can bring side effects, so it is a managed measure, not a permanent cure; oil change and on-site treatment have their own efficiency limits and consequences. The brochure's value is that it documents what happens over years, not just at commissioning — exactly the horizon an asset owner cares about.
How we use it
When corrosive sulphur turns up in a client's transformer, we use TB 625 as the backbone of the advice. It informs how we weigh the risk factors for a specific unit, how we frame the choice between passivation, treatment, and oil change, and how we set the monitoring that follows any intervention. It also explains second-order effects — such as gassing behaviour after passivator addition — that can muddy gas-in-oil interpretation. We treat it as the bridge between the laboratory detection methods and a sound, long-term management plan, not as a substitute for the underlying test standards or for unit-specific engineering judgement.